Article

Self-Assembled Molecular-Electronic Films Controlled by Room Temperature Quantum Interference

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Abstract

If single-molecule, room-temperature, quantum interference (QI) effects could be translated into massively parallel arrays of molecules located between planar electrodes, QI-controlled molecular transistors would become available as buildingblocks for future electronic devices. Here, we demonstrate unequivocal signatures of room-temperature QI in vertical tunneling transistors, formed from self-assembled monolayers (SAMs), with stable room-temperature switch- ing operations. As a result of constructive QI effects, the conductances of the junctions formed from anthanthrene-based molecules with two different connectivities differ by a factor of 34, which can further increase to 173 by controlling the molecule-electrode interface with different terminal groups. Field-effect control is achieved using an ionic liquid gate, whose strong vertical electric field penetrates through the graphene layer and tunes the energy levels of the SAMs. The resulting room-temperature on-off current ratio of the lowest-conductance SAMs can reach up to 306, about one order of magnitude higher than that of the highest-conductance SAMs.

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... INTRODUCTION Charge transport through single or self-assembled monolayer (SAM) of molecules has attracted great interest in the past two decades 1-8 due to the potential applications of molecular tunnel junctions beyond current scaling limits as diodes, [9][10][11][12] switches, [13][14][15][16] and transistors. 17,18 Furthermore, charge tunneling transport underpins basic physical and chemical mechanisms at the molecular level, such as room-temperature quantum interference (QI) effects, [19][20][21][22] thermoelectricity, 23,24 and dynamic chemical processes. [25][26][27][28] Traditionally, SAM-based molecular tunnel junctions were constructed with functional SAMs sealed within parallel-plane electrodes, [29][30][31][32][33] such as ferrocene (Fc) SAMs between an ultra-flat bottom electrode and an EGaIn top electrode. ...
... Therefore, there is an urgent need to develop new ways to realize the in situ control of the redox states and molecule/electrode coupling of SAM-based molecular tunnel junctions. Recently, we have demonstrated a new design of vertical molecular tunnel junctions based on Au/SAM/graphene heterostructures, 21,22 where single-layer graphene (SLG) acts as ...
... The vertical molecular tunnel junction (Figure 2A) is constructed as described in our previous works. 21,22 Specifically, an ultra-flat Au film is deposited on the surface of highly doped silicon in a small hole at the center of a silicon/SiO 2 chip, where the conducting silicon can be used as source electrode for the final device. To incorporate the Fc-SAM, a monolayer of 6-ferrocenylhexanethiol (FcC 6 S) is then self-assembled on the surface of Au film and confirmed by X-ray photoelectron spectroscopy (XPS) and electrochemical characterization 42 (Figures S1 and S3). ...
Article
Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).
... In another context, Famili et al. fabricated an electrochemical FET device by using diethyl methyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) IL on the single-layer GR, which was deposited on the self-assembled monolayers (SAMs) and gold channel surface to make a source channel. Schematic for the setup of the device with a vertical IL gate through the GR layer is illustrated in Figure 11D-F [150]. ...
... (D-F) Schematic illustration for the setup of the device with a vertical IL gate through GR layer to SAMs and molecular structures for DEME + cation and TFSIanion. Reprinted with permission from Ref.[150]. Copyright 2019, Elsevier. ...
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Ionic liquids are gaining high attention due to their extremely unique physiochemical properties and are being utilized in numerous applications in the field of electrochemistry and bio-nanotechnology. The excellent ionic conductivity and the wide electrochemical window open a new avenue in the construction of electrochemical devices. On the other hand, carbon nanomaterials, such as graphene (GR), graphene oxide (GO), carbon dots (CDs), and carbon nanotubes (CNTs), are highly utilized in electrochemical applications. Since they have a large surface area, high conductivity, stability, and functionality, they are promising in biosensor applications. Nevertheless, the combination of ionic liquids (ILs) and carbon nanomaterials (CNMs) results in the functional ILs-CNMs hybrid nanocomposites with considerably improved surface chemistry and electrochemical properties. Moreover, the high functionality and biocompatibility of ILs favor the high loading of biomolecules on the electrode surface. They extremely enhance the sensitivity of the biosensor that reaches the ability of ultra-low detection limit. This review aims to provide the studies of the synthesis, properties, and bonding of functional ILs-CNMs. Further, their electrochemical sensors and biosensor applications for the detection of numerous analytes are also discussed.
... [13][14][15][16] Meanwhile, molecular materials also have advantages in assembly, from macro/nano single crystals and selfassembled monolayers to single molecules, which provides the basis for constructing FETs from macroscale to single molecule. [17][18][19][20] In particular, the design and construction of FETs with single molecules can satisfy the need of miniaturization and functionalization of electronic applications. Due to the inherent molecular size of single-molecule FETs, it can overcome the development bottleneck of Moore's Law. ...
Article
Molecule‐based field‐effect transistors (FETs) are of great significance as they have a wide range of application prospects, such as logic operations, information storage and sensor monitoring. This account mainly introduces and reviews our recent work in molecular FETs. Specifically, through molecular and device design, we have systematically investigated the construction and performance of FETs from macroscale to nanoscale and even single molecule. In particular, we have proposed the broad concept of molecular FETs, whose functions can be achieved through various external controls, such as light stimulation, and other physical, chemical or biological interactions. In the end, we tend to focus the discussion on the development challenges of single‐molecule FETs, and propose prospects for further breakthroughs in this field. This personal account introduces our recent works in the construction and performance of molecule‐based FETs from macroscale to nanoscale and even single molecule.
... 20,[42][43][44] Further, in another report, the combination of single-electron transistors with local gates provides a unique way of charge interaction with an external molecule observed in terms of quantum transport behavior of the system. 45,46 However, electrical manipulation at room temperature is difficult due to the thermal excitation energy (k B T) of the carriers. So far, precise carrier confinement is either enabled through the various arrangement of TMDC layers or controlling the carrier transport by a narrow gating to sense small perturbations. ...
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Combined with diverse two-dimensional (2D) materials for semiconductor interfaces are attractive for electrically controllable carrier confinement to enable excellent electrostatic control. We investigated the transport characteristic in heterointerface of multilayer molybdenum disulfide and hexagonal boron nitride (MoS2/h-BN) to reveal that the charge transfer switching (CTS) is highly dependent on both the local gate constriction and bias applied across the channel. Notably, the CTS is controlled at a molecular level through electrotunable gated constriction. The resulting significant change in conductance due to exposing 100 parts-per-billion of nitrogen dioxide molecules led to a high on/off ratio of 10² for completely switching off the channel thus, acting as a molecular switch. First-principle calculations further explained the mechanism of molecular CTS in the device. The molecular tunability of CTS has not been previously reported in any of the hetero semiconductor interfaces. Our finding opens avenues to exploit various atomically thin heterostructures for the mesoscopic transport phenomena towards molecular switching operation at room temperature.
... 20,[42][43][44] Further, in another report, the combination of single-electron transistors with local gates provides a unique way of charge interaction with an external molecule observed in terms of quantum transport behavior of the system. 45,46 However, electrical manipulation at room temperature is difficult due to the thermal excitation energy (k B T) of the carriers. So far, precise carrier confinement is either enabled through the various arrangement of TMDC layers or controlling the carrier transport by a narrow gating to sense small perturbations. ...
Preprint
Full-text available
Combined diverse two-dimensional (2D) materials for semiconductor interfaces are attractive for electrically controllable carrier confinement to enable excellent electrostatic control. We investigated the transport characteristic in heterointerface of multilayer molybdenum disulfide and hexagonal boron nitride (MoS$_2$/h-BN) to reveal that the charge transfer switching (CTS) is highly dependent on both the local gate constriction and bias in the channel. Notably, the CTS is shown to be controlled at a molecular level through electrotunable gated constriction. The resulting significant change in conductance due to exposing 100 parts-per-billion of nitrogen dioxide gas led to a high on/off ratio of 10 2 for completely switching off the channel thus, acting as a molecular switch. First-principle calculations further explained the mechanism of molecular CTS in the device. The molecular tunability of CTS has not been previously reported in any of the van der Waals semiconductor interfaces. Our finding opens avenues to exploit various atomically thin heterostructures for the mesoscopic transport phenomena towards molecular switching operation at room temperature.
... The wide variety of currently available two-dimensional (2D) materials has enabled the stacking of different atomic layers to yield new electronic materials held together by van der Waals (vdW) forces (1)(2)(3)(4)(5). Despite their early promise (6), the preparation of defect-free 2D-vdW heterojunctions (2D-vdWHs) remains a challenge. ...
Article
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Two-dimensional van der Waals heterojunctions (2D-vdWHs) stacked from atomically thick 2D materials are predicted to be a diverse class of electronic materials with unique electronic properties. These properties can be further tuned by sandwiching monolayers of planar organic molecules between 2D materials to form molecular 2D-vdWHs (M-2D-vdWHs), in which electricity flows in a cross-plane way from one 2D layer to the other via a single molecular layer. Using a newly developed cross-plane break junction technique, combined with density functional theory calculations, we show that M-2D-vdWHs can be created and that cross-plane charge transport can be tuned by incorporating guest molecules. The M-2D-vdWHs exhibit distinct cross-plane charge transport signatures, which differ from those of molecules undergoing in-plane charge transport.
... In particular, a growing number of studies have identified quantum interference effects in the thermoelectric properties of single molecules . More recently, it has been demonstrated that these single-molecule QI effects can be translated into self-assembled monolayers [146][147][148][149][150][151][152][153][154][155][156][157][158], thereby creating two-dimensional materials, whose electronic and thermoelectric properties are controlled by room-temperature quantum interference. Interest in molecular-scale thermoelectricity has also stimulated studies of thermal transport through single molecules [159][160][161][162][163][164][165][166][167][168][169][170][171], where room-temperature phonon interference may provide a route to suppressing thermal conductance and increasing the thermoelectric performance of molecular-scale devices and materials. ...
... 294 Jia et al. 68 incorporated SAMs of the through-space molecular wires into transistors, demonstrating the gating of destructive QI features. Famili et al. 295 used the same strategy to gate QI effects in MEJs arising from bond topology [ Fig. 15(b)]. Carlotti et al. 296 further explored many derivatives of AQ molecular wires, demonstrating that the degree of current suppression can be controlled by including electron-donating and -withdrawing functional groups. ...
Article
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This review focuses on molecular ensemble junctions in which the individual molecules of a monolayer each span two electrodes. This geometry favors quantum mechanical tunneling as the dominant mechanism of charge transport, which translates perturbances on the scale of bond lengths into nonlinear electrical responses. The ability to affect these responses at low voltages and with a variety of inputs, such as de/protonation, photon absorption, isomerization, oxidation/reduction, etc., creates the possibility to fabricate molecule-scale electronic devices that augment; extend; and, in some cases, outperform conventional semiconductor-based electronics. Moreover, these molecular devices, in part, fabricate themselves by defining single-nanometer features with atomic precision via self-assembly. Although these junctions share many properties with single-molecule junctions, they also possess unique properties that present a different set of problems and exhibit unique properties. The primary trade-off of ensemble junctions is complexity for functionality; disordered molecular ensembles are significantly more difficult to model, particularly atomistically, but they are static and can be incorporated into integrated circuits. Progress toward useful functionality has accelerated in recent years, concomitant with deeper scientific insight into the mediation of charge transport by ensembles of molecules and experimental platforms that enable empirical studies to control for defects and artifacts. This review separates junctions by the trade-offs, complexity, and sensitivity of their constituents; the bottom electrode to which the ensembles are anchored and the nature of the anchoring chemistry both chemically and with respect to electronic coupling; the molecular layer and the relationship among electronic structure, mechanism of charge transport, and electrical output; and the top electrode that realizes an individual junction by defining its geometry and a second molecule–electrode interface. Due to growing interest in and accessibility of this interdisciplinary field, there is now sufficient variety in each of these parts to be able to treat them separately. When viewed this way, clear structure–function relationships emerge that can serve as design rules for extracting useful functionality.
... Furthermore, we show that the thermoelectrical performance of anthracene-based molecular films can be boosted by a judicious choice of connectivity to electrodes, combined with an optimal choice of terminal groups. Although the effect of CQI on the electrical conductance of SAMs was reported only recently, 58 the above demonstration of CQIcontrolled molecular films is the first report of CQI-boosted thermoelectricity. It opens the way to new design strategies for functional ultra-thin-film thermoelectric materials and electronic building blocks for future integrated circuits. ...
Article
Full-text available
The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be con-trolled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on constructive QI effects within the core. We also demonstrate that for molecules with thioether anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ~50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step towards functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.
... For the anthanthrene core of Figure 1, M 3,12 = 9 and M 9,22 = 1. Hence their conductance ratio is predicted to be |M 3,12 | 2 /|M 9,22 | 2 = 81, which is close to the measured value of the conductance ratio, both for single molecules and for self-assembled monolayers [12,23,43]. ...
Preprint
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We study the quantum interference (QI) effects in three-terminal Andreev interferometers based on polyaromatic hydrocarbons (PAH's) under non-equilibrium conditions. The Andreev interferometer consists of a PAH coupled to two superconducting and one normal conducting terminals. We calculate the current measured in the normal lead as well as the current between the superconducting terminals under non-equilibrium conditions. We show that both the QI arising in the PAH cores and the bias voltage applied to a normal contact have a fundamental effect on the charge distribution associated with the Andreev Bound States (ABS's). QI can lead to a peculiar dependence of the normal current on the superconducting phase difference that was not observed in earlier studies of mesoscopic Andreev interferometers. We explain our results by an induced asymmetry in the spatial distribution of the electron- and hole-like quasiparticles. The non-equilibrium charge occupation induced in the central PAH core can result in a $\pi$ transition in the current-phase relation of the supercurrent for large enough applied bias voltage on the normal lead. The asymmetry in the spatial distribution of the electron- and hole-like quasiparticles might be used to split Cooper pairs and hence to produce entangled electrons in four terminal setups.
... Gold is an important material and used as a reference for molecular electronic studies [87,139]. Most of the theoretical works for developing atomic-scale electronic components are performed considering gold as the electrode material [108,122,[139][140][141][142][143][144]. Apart from this, the properties and structures of gold in different orientations have been studied in detail by many researchers both in ultra-high vacuum (UHV) and electrochemical environments [145]. ...
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Manufacturing at the atomic scale is the next generation of the industrial revolution. Atomic and close-to-atomic scale manufacturing (ACSM) helps to achieve this. Atomic force microscopy (AFM) is a promising method for this purpose since an instrument to machine at this small scale has not yet been developed. As the need for increasing the number of electronic components inside an integrated circuit chip is emerging in the present-day scenario, methods should be adopted to reduce the size of connections inside the chip. This can be achieved using molecules. However, connecting molecules with the electrodes and then to the external world is challenging. Foundations must be laid to make this possible for the future. Atomic layer removal, down to one atom, can be employed for this purpose. Presently, theoretical works are being performed extensively to study the interactions happening at the molecule–electrode junction, and how electronic transport is affected by the functionality and robustness of the system. These theoretical studies can be verified experimentally only if nano electrodes are fabricated. Silicon is widely used in the semiconductor industry to fabricate electronic components. Likewise, carbon-based materials such as highly oriented pyrolytic graphite, gold, and silicon carbide find applications in the electronic device manufacturing sector. Hence, ACSM of these materials should be developed intensively. This paper presents a review on the state-of-the-art research performed on material removal at the atomic scale by electrochemical and mechanical methods of the mentioned materials using AFM and provides a roadmap to achieve effective mass production of these devices.
... These correlation effects are scientifically highly interesting and are already under study in the scientific community, with research groups trying for example to add side groups to the molecular backbone as anchor points to promote ordered crosslinking without altering the thermoelectric properties and preserve quantum features. 22 Currently, thermal radiation in the so-called extreme near-field across gaps of only few nanometers is highly debated in the community 23,24 . Experimental data suggests that the presence of molecular systems may have significant influence and could reach similar orders of magnitude as the phonon conduction across the molecules (~nW/K for a sharp tip a few nanometers far from a surface) 23 . ...
Preprint
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Molecules have the potential to act as sharp energy filters for electrical currents and could thereby outperform other materials considered for thermoelectric energy conversion. Based on this vision, a considerable amount of research is being performed worldwide. However, there is a large discrepancy between predictions and actual demonstrations in the literature, and a research roadmap is needed to highlight necessary steps to transition from fundamental research into a viable technology. Considering both scientific and technological challenges, we propose eight milestones on the way yet to go for technological applications. The multi-disciplinary knowledge, generated while addressing the technological challenges, can yield to novel applications and answer unresolved fundamental science questions, which are of interest far beyond mere energy conversion.
... [162] Furthermore, conductivity is significantly higher along the short axis than along the long axis. [165,166] Consequently, functionalization at positions 4 and 10 is suitable to tune solubility and stacking, whereas positions 6 and 12 are functionalized in order to tune the electronic and optical properties. [162] In summary, functionalization transforms anthanthrene into a photostable, reversibly oxidizable and/or reducible material with good charge transport properties and a suitable band gap for the use in molecular electronics and solar energy conversion applications, such as OFET, OLED, organic solar cells (OSC), and DSSCs. ...
Thesis
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Today’s climate is undeniably affected by human influences. The enhanced emission of greenhouse gasses started with early settlements, increased dramatically during the industrial revolution, and is still proceeding. One way to reduce greenhouse gas emission is the promotion of renewable energies. The most abundant energy source available on earth is sunlight, which can be converted into electricity via solar cells. All-carbon solar cells are considered as a cheap and environmentally friendly energy source. Alongside manifold possible applications due to their extraordinary physical and optical properties, single wall carbon nanotubes (SWCNT) play an important role in this kind of solar cells, in which they serve as a conductor or light absorbing material. Many applications of SWCNTs benefit from tailored properties of SWCNTs, as well as individualized and chirally sorted SWCNTs. A way to approach this, without compromising the intrinsic properties of SWCNTs, is non-covalent functionalization. In this work, novel photoactive supramolecular assemblies of SWCNTs and molecular dyes are presented. Thorough characterization was guaranteed by spectroscopy, microscopy, and electrochemistry. In particular, steady state absorption and fluorescence spectroscopy, excitation spectroscopy, time-correlated single photon counting, transient absorption spectroscopy, Raman mapping, atomic force microscopy, transmission electron microscopy, cyclic voltammetry, differential pulse voltammetry, square wave voltammetry, and spectroelectrochemistry were applied. Of particular interest were investigations on debundling and/or individualization of SWCNTs, doping of SWCNTs, and possible energy and/or charge transfer between both SWCNTs and dyes. In addition, chiral selectivity and its relation to the molecular structure of the dyes was investigated. The flat or twisted π-systems are either located directly on top of SWCNTs or are immobilized by means of a pyrene anchor. Furthermore, SWCNTs are wrapped by oligomeric π-systems or interlocked by a ring of linked π-systems. The successful formation of the supramolecular assemblies in dispersion was ascertained for all investigated systems. In the case of individual π-systems on SWCNTs, three water soluble perylenediimides with an increasing number of bromine substituents at the bay positions were interfaced with SWCNTs. This substitution pattern induces different twist angles in the perylenediimides and different electron accepting character. In addition, the variation in substitution leads to different absorption, fluorescence, excited state dynamics, and aggregation behavior. In supramolecular assemblies with SWCNTs in D2O, the stability in dispersion, the doping of SWCNTs, electronic interactions, and chiral selectivity also vary between the assemblies with differently substituted perylenediimides. Photoinduced charge separation and hole migration ii in SWCNTs were observed in all assemblies. Other examples for individual π-systems on SWCNTs are two asymmetrically substituted, alkylated zinc porphyrins featuring an amphiphilic character. Their aggregation behavior in THF:water (1:1 v/v) changed significantly in assemblies with SWCNTs. Moreover, photoinduced charge separation was observed. In addition, free base and zinc porphyrins with bulky substituents at their meso-position were anchored onto SWCNTs via pyrene anchors. These anchors consisted of either one or three pyrenes. It was shown that the bulky substituents, the geometry of the linker, and the number of pyrenes affect the properties of supramolecular assemblies with nanocarbons. Whereas no direct interaction or charge transfer between the porphyrins and SWCNTs was observed in dispersions, doping was verified in the solid state. Furthermore, oligomeric, alkylated anthanthrene molecules wrapped around SWCNTs in DMF and ethanol were investigated. Upon formation of the assembly with SWCNTs, the oligomeric anthanthrenes changed their characteristics related to aggregation and excimer-like structures. Besides electronic interactions in the ground and excited state, energy transfer upon photoexcitation was confirmed. Turning to the mechanically interlocked SWCNTs, the investigated interlocked ring around SWCNTs consists of zinc porphyrins. Pronounced chiral selectivity with respect to the diameter of the SWCNTs was corroborated upon formation of this very stable assembly. In addition, doping of the SWCNTs and photoinduced charge transfer was verified.
... Although there are many experimental investigations of charge transport through bulk perovskite materials, including thin films 10 , nanocrystals 11 , and single crystals 12 , investigations at the nanoscale, to reveal QI effects in their room-temperature transport properties remain as a major experimental challenge. The extensions of singlemolecule charge transport measurements from conjugated molecular families 13 to molecular assemblies 14,15 , clusters 16 , and the recently developed Au-halogen interfacial engineering 17 offer an opportunity to gain an insight into microscopic charge transport through Ångstrom-scale perovskite materials. ...
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The studies of quantum interference effects through bulk perovskite materials at the Ångstrom scale still remain as a major challenge. Herein, we provide the observation of room-temperature quantum interference effects in metal halide perovskite quantum dots (QDs) using the mechanically controllable break junction technique. Single-QD conductance measurements reveal that there are multiple conductance peaks for the CH3NH3PbBr3 and CH3NH3PbBr2.15Cl0.85 QDs, whose displacement distributions match the lattice constant of QDs, suggesting that the gold electrodes slide through different lattice sites of the QD via Au-halogen coupling. We also observe a distinct conductance ‘jump’ at the end of the sliding process, which is further evidence that quantum interference effects dominate charge transport in these single-QD junctions. This conductance ‘jump’ is also confirmed by our theoretical calculations utilizing density functional theory combined with quantum transport theory. Our measurements and theory create a pathway to exploit quantum interference effects in quantum-controlled perovskite materials.
... Under the superposition of QI effects and molecule-electrode interface effects, the switching on/off ratio for the lowest conductance SAM can reach up to 306, which is an order of magnitude higher than that of molecule without adjustment. In a word, it is very effective and feasible to improve the performance of molecular devices based on the QI effects [412]. ...
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Single-molecule optoelectronic devices promise a potential solution for miniaturization and functionalization of silicon-based microelectronic circuits in the future. For decades of its fast development, this field has made significant progress in the synthesis of optoelectronic materials, the fabrication of single-molecule devices and the realization of optoelectronic functions. On the other hand, single-molecule optoelectronic devices offer a reliable platform to investigate the intrinsic physical phenomena and regulation rules of matters at the single-molecule level. To further realize and regulate the optoelectronic functions toward practical applications, it is necessary to clarify the intrinsic physical mechanisms of single-molecule optoelectronic nanodevices. Here, we provide a timely review to survey the physical phenomena and laws involved in single-molecule optoelectronic materials and devices, including charge effects, spin effects, exciton effects, vibronic effects, structural and orbital effects. In particular, we will systematically summarize the basics of molecular optoelectronic materials, and the physical effects and manipulations of single-molecule optoelectronic nanodevices. In addition, fundamentals of single-molecule electronics, which are basic of single-molecule optoelectronics, can also be found in this review. At last, we tend to focus the discussion on the opportunities and challenges arising in the field of single-molecule optoelectronics, and propose further potential breakthroughs.
... 4-Ethynylthioanisole, 4-(ethynyl)phenyl-tert-butylthioether and 1,5-dibromoanthracene were synthesised through adapted literature procedures. 10,29,54 Solvents used in reactions were collected from solvent towers sparged with nitrogen and dried with 3 Å molecular sieves, apart from DIPA, which was distilled on to activated 3 Å molecular sieves. TS gold preparation for SPM: A Si wafer (5 mm x 5 mm) was cleaned in an ultra-sonication bath with acetone, methanol and isopropanol in series, before cleaning with oxygen plasma for 5 minutes. ...
Preprint
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The realization of self-assembled molecular-electronic films, whose room-temperature transport properties are controlled by quantum interference (QI), is an essential step in the scale-up QI effects from single molecules to parallel arrays of molecules. Recently, the effect of destructive QI (DQI) on the electrical conductance of self-assembled monolayers (SAMs) has been investigated. Here, through a combined experimental and theoretical investigation, we demonstrate chemical control of different forms of constructive QI (CQI) in cross-plane transport through SAMs and assess its influence on cross-plane thermoelectricity in SAMs. It is known that the electrical conductance of single molecules can be controlled in a deterministic manner, by chemically varying their connectivity to external electrodes. Here, by employing synthetic methodologies to vary the connectivity of terminal anchor groups around aromatic anthracene cores, and by forming SAMs of the resulting molecules, we clearly demonstrate that this signature of CQI can be translated into SAM-on-gold molecular films. We show that the conductance of vertical molecular junctions formed from anthracene-based molecules with two different connectivities differ by a factor of approximately 16, in agreement with theoretical predictions for their conductance ratio based on constructive QI effects within the core. We also demonstrate that for molecules with thiol anchor groups, the Seebeck coefficient of such films is connectivity dependent and with an appropriate choice of connectivity can be boosted by ~50%. This demonstration of QI and its influence on thermoelectricity in SAMs represents a critical step towards functional ultra-thin-film devices for future thermoelectric and molecular-scale electronics applications.
... For the anthanthrene core of Figure 1, M 3,12 = 9 and M 9,22 = 1. Hence their conductance ratio is predicted to be |M 3,12 | 2 /|M 9,22 | 2 = 81, which is close to the measured value of the conductance ratio, both for single molecules and for self-assembled monolayers [12,23,43]. ...
Article
Full-text available
We study the quantum interference (QI) effects in three-terminal Andreev interferometers based on polyaromatic hydrocarbons (PAHs) under non-equilibrium conditions. The Andreev interferometer consists of a PAH coupled to two superconducting and one normal conducting terminals. We calculate the current measured in the normal lead as well as the current between the superconducting terminals under non-equilibrium conditions. We show that both the QI arising in the PAH cores and the bias voltage applied to a normal contact have a fundamental effect on the charge distribution associated with the Andreev Bound States (ABSs). QI can lead to a peculiar dependence of the normal current on the superconducting phase difference that was not observed in earlier studies of mesoscopic Andreev interferometers. We explain our results by an induced asymmetry in the spatial distribution of the electron- and hole-like quasiparticles. The non-equilibrium charge occupation induced in the central PAH core can result in a π transition in the current-phase relation of the supercurrent for large enough applied bias voltage on the normal lead. The asymmetry in the spatial distribution of the electron- and hole-like quasiparticles might be used to split Cooper pairs and hence to produce entangled electrons in four terminal setups.
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We report measurements on gold|single-molecule|gold junctions, using a modified scanning tunneling microscope-break junction (STM-BJ) technique, of the Seebeck coefficient and electrical conductance of a series of bridged biphenyl molecules, with meta connectivities to pyridyl anchor groups. These data are compared with a previously reported study of para-connected analogues. In agreement with a tight binding model, the electrical conductance of the meta series is relatively low and is sensitive to the nature of the bridging groups, whereas in the para case the conductance is higher and relatively insensitive to the presence of the bridging groups. This difference in sensitivity arises from the presence of destructive quantum interference in the π system of the unbridged aromatic core, which is alleviated to different degrees by the presence of bridging groups. More precisely, the Seebeck coefficient of meta-connected molecules was found to vary between -6.1 μV/K and -14.1 μV/K, whereas that of the para-connected molecules varied from -5.5 μV/K and -9.0 μV/K.
Article
Stacking interactions are of significant importance in the fields of chemistry, biology, and material optoelectronics because they determine the efficiency of charge transfer between molecules and their quantum states. Previous studies have proven that when two monomers are π-stacked in series to form a dimer, the electrical conductance of the dimer is significantly lower than that of the monomer. Here, we present a strong opposite case that when two anthanthrene monomers are π-stacked to form a dimer in a scanning tunneling microscopic break junction, the conductance increases by as much as 25 in comparison with a monomer, which originates from a room-temperature quantum interference. Remarkably, both theory and experiment consistently reveal that this effect can be reversed by changing the connectivity of external electrodes to the monomer core. These results demonstrate that synthetic control of connectivity to molecular cores can be combined with stacking interactions between their π systems to modify and optimize charge transfer between molecules, opening up a wide variety of potential applications ranging from organic optoelectronics and photovoltaics to nanoelectronics and single-molecule electronics.
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We study the role of electronic spin and valley symmetry in the quantum interference (QI) patterns of the transmission function in graphene quantum junctions. In particular, we link it to the position of the destructive QI antiresonances. When the spin or valley symmetry is preserved, electrons with opposite spin or valley display the same interference pattern. On the other hand, when a symmetry is lifted, the antiresonances are split, with a consequent dramatic differentiation of the transport properties in the respective channel. We demonstrate rigorously this link in terms of the analytical structure of the electronic Green function, which follows from the symmetries of the microscopic model, and we confirm the result with numerical calculations for graphene nanoflakes. We argue that this is a generic and robust feature that can be exploited in different ways for the realization of nanoelectronic QI devices, generalizing the recent proposal of a QI-assisted spin-filtering effect [A. Valli et al., Nano Lett. 18, 2158 (2018)].
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As stated in the classic Kirchhoff's circuit laws, the total conductance of two parallel channels in an electronic circuit is the sum of the individual conductance. However, in molecular circuits, the quantum interference (QI) between the individual channels may lead to apparent invalidity of Kirchhoff's laws. Such an effect can be very significant in single-molecule circuits consisting of partially overlapped multiple transport channels. Herein, an investigation on how the molecular circuit conductance correlates to the individual channels is conducted in the presence of QI. It is found that the conductance of multi-channel circuit consisting of both constructive and destructive QI is significantly smaller than the addition of individual ones due to the interference between channels. In contrast, the circuit consisting of destructive QI channels exhibits an additive transport. These investigations provide a new cognition of transport mechanism and manipulation of transport in multi-channel molecular circuits.
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Heterostructures with designable electronic interfaces represent the material foundation for modern electronic and optoelectronic devices. The conventional heterostructures rely on covalent bonds to integrate the constituent materials with strict lattice-matching requirements. The use of van der Waals (VDW) force allows a bond-free strategy to integrate a wide range of materials, including zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanowires, two-dimensional (2D) nanosheets, and three-dimensional (3D) bulk materials, beyond the reach of conventional heterostructures, creating versatile artificial VDW heterostructures with nearly arbitrary modulation of chemical compositions and electronic structures by design. In this review, we start with a brief review of the unique attributes and merits of VDW heterostructures and then highlight a series of example heterostructures assembled from various low-dimensional materials, including 1D/1D, 0D/2D, 1D/2D, 2D/2D, 2D/3D, and 3D-3D heterojunctions and devices. We will conclude with a prospect on the new opportunities and emerging challenges arising in these unconventional heterostructures.
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The technique of self-assembled monolayers (SAMs) is frequently applied for grafting functional groups or area-selective deposition of thin films on a material surface. The formation and quality of SAMs are fundamentally determined by thermodynamic data, which are difficult to measure with available experimental methods. This work quantitatively extracted thermodynamic parameters including ΔH°, ΔG°, and ΔS° during the SAMs construction process with an ultrasensitive resonant microcantilever as molecule-surface interactions real-time recording tool. By correlating the thermodynamic parameters with self-assembling temperatures, a new thermodynamic phase-like transition effect of molecular self-assembly has been first revealed. The sharp transition of the thermodynamic parameters defines the critical condition for SAMs formation. The thermodynamic data further provide optimized reaction conditions for constructing high-quality SAMs. The explored quantitative thermodynamic analysis method not only plays as criterion for SAM growth but also helps to fundamentally elucidate physicochemical mechanism of spontaneous self-assembly.
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The biological template and its mutants have vital significance in next generation remediation, electrochemical, photovoltaic, catalytic, sensing and digital memory devices. However, a microscopic model describing the biotemplating process is generally lacking on account of modelling complexity, which has prevented widespread commercial use of biotemplates. Here, we demonstrate M13-biotemplating kinetics in atomic resolution by leveraging large-scale molecular dynamics (MD) simulations. The model reveals the assembly of gold nanoparticles on two experimentally-based M13 phage types using full M13-capsid structural models and with polarizable gold nanoparticles in explicit solvent. Both mechanistic and structural insights into the selective binding affinity of the M13 phage to gold nanoparticles are obtained based on a previously unconsidered clamp-based binding-pocket-favored N-terminal-domain assembly and also on surface–peptide flexibility. These results provide a deeper level of understanding of protein sequence-based affinity and open the route for genetically engineering a wide range of 3D electrodes for high-density low-cost device integration.
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Electron transport properties of polycyclic truxene derivatives have been investigated by the single molecule conductance measurement technique and theoretical study. Molecules with nitrogen and carbonyl substituents at the bridge sites exhibit higher single-molecule conductances by almost one order of magnitude compared with non-substituted analogues. It can be ascribed that the anti-resonance feature produced by destructive quantum interference (DQI) is alleviated and pushed away from the Fermi energy. These findings provide an effective chemical strategy for manipulating the DQI behavior in single molecular devices.
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Understanding and controlling the orbital alignment of molecules placed between electrodes is essential in the design of practically-applicable molecular and nanoscale electronic devices. The orbital alignment is highly determined by...
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In this work, the adsorption and orientation on gold nanoparticles (AuNps) of a new family of cruciform systems consisting of thiophene rings and imino groups were studied. The structural modification and its influence on the adsorbate‐substrate interaction were evaluated by UV–Vis spectroscopy and Surface Enhanced Raman Spectroscopy (SERS). The absence of SERS spectrum for (N,N′‐bis(4‐(trifluoromethyl)benzylidene)‐2,5‐di (thiophene‐2‐yl)‐1,4‐diaminobenzene) CFF shows that the inclusion of a trifluoromethyl group (‐CF3) on the benzylidene fragment limits the interaction of the CFF system with the gold substrate, in contrast, to that obtained for (N, N′‐dibenzylidene‐2,5‐di (thiophene‐2‐yl) ‐1,4‐diaminobenzene) 2‐CF and (N, N′‐bis (4‐methoxybenzylidene) ‐2,5‐di (thiophene‐2yl) ‐1,4‐diaminobenzene) CMF, where the adsorption took place preferentially through the thiophene rings, resulting in partial quinoidization. On the other hand, the interaction for compound (N, N′‐bis (4‐methylenepyridinyl) ‐2,5‐di (thiophene‐2‐yl) ‐1,4‐diaminobenzene) CPy with the surface was conducted by means of the pyridinic fragments. The systematic modification of the bifunctional cruciform systems, with groups of different nature, makes it possible to rationalize the structural aspects that directly influence the adsorbate‐substrate interaction and molecular orientation on gold substrates. These structural parameters are the basis to the development of stable molecular assemblies, which can act as basic building blocks in the manufacture of molecular switches. The adsorption and orientation of a new family of cruciform systems were evaluated by Surface Amplified Raman Spectroscopy (SERS). In this sense, the structural modifications allowed modulating the substrate‐adsorbate interaction center, affinity, and preferential orientation of the adsorbates with respect to the gold nanoparticles.
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Using individual molecules as conducting bridges for electrons offers opportunities when investigating quantum phenomena that are not readily accessible from experiments involving ensembles of molecules. The probing of single molecules has led, over the past few decades, to the rise of molecular electronics. Although single-supermolecule electronics is an emerging field, it is not yet a well-defined area of molecular electronics. There is little doubt, however, that single-supermolecule electronics is poised to have an impact on molecular electronics for the simple reason that non-covalent interactions between molecular components in complexes have a profound effect on electron conductivities. In this Review, we survey this emerging field from the standpoint of non-covalent interactions in mechanically interlocked molecules, as well as in supermolecules, and discuss the (super)structure–property relationship of four different interactions associated with (supra)molecular junctions. They are host–guest interactions, hydrogen bonding, π–π interactions, and non-covalent interactions present in mechanically interlocked molecules. We focus our attention on providing a supramolecular-level understanding of charge transport behaviour associated with each interaction, as well as demonstrating the theoretical background and experimental readiness of single-supermolecule electronics for potential applications, such as nucleic acid and peptide sequencing, and the design and production of quantum interference devices, random-access memories and integrated devices.
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Oligo(arylene ethynylene) (OAE) derivatives are the "workhorse" molecules of molecular electronics. Their ease of synthesis and flexibility of functionalisation mean that a diverse array of OAE molecular wires have been designed, synthesised and studied theoretically and experimentally in molecular junctions using both single-molecule and ensemble methods. This review summarises the breadth of molecular designs that have been investigated with emphasis on structure-property relationships with respect to the electronic conductance of OAEs. The factors considered include molecular length, connectivity, conjugation, (anti)aromaticity, heteroatom effects and quantum interference (QI). Growing interest in the thermoelectric properties of OAE derivatives, which are expected to be at the forefront of research into organic thermoelectric devices, is also explored.
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The continued scaling of silicon-based electronics is quickly approaching its fundamental material limit, which has motivated worldwide efforts in exploring new electronic materials and unconventional device architectures. In particular, two-dimensional (2D) atomic crystals, halide perovskites, and self-assembled molecular monolayers represent prominent examples of emerging electronic materials with significant promise for the continued miniaturization or function diversification. However, probing the fundamental transport properties and capturing the intrinsic merits of these emerging electronic materials are not always straightforward because they are usually delicate and prone to degradation during the conventional material integration and device fabrication steps. To this end, an alternative bond-free integration strategy, in which the pre-synthesized/fabricated material components are physically transferred and assembled together through weak van der Waals (vdW) force, offers a mild process for seamlessly combining highly disparate materials to form artificial heterostructures with atomically clean and electronically sharp interfaces, enabling the creation of high-performance devices for probing and pushing the limit of the emerging electronic materials. In this article, we summarize the efforts in our laboratory over the past 12 years, in developing, optimizing, and expanding the vdW integration strategy for creating and investigating high-performance devices from 2D atomic crystals, soft lattice halide perovskites, and self-assembled molecular monolayers. We also discuss how flexible vdW integration may enable a new generation of artificial materials, including vdW superlattices, vdW thin films or 3D frameworks. We conclude with a brief prospect on the critical challenges and emerging opportunities arising with this unique material integration strategy.Graphic abstract
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Recent years have witnessed the fabrication of various non-covalent interaction-based molecular electronic devices. In the non-covalent interaction-based molecular devices, the strength of the interfacial coupling between molecule and electrode is weakened compared to that of the covalent interaction-based molecular devices, which provides wide applications in fabricating versatile molecular devices. In this review, we start with the methods capable of fabricating graphene-based nanogaps, and the following routes to construct non-covalent interaction-based molecular junctions with graphene electrodes. Then we give an introduction to the reported non-covalent interaction-based molecular devices with graphene electrodes equipped with different electrical functions. Moreover, we summarize the recent progress in the design and fabrication of new-type molecular devices based on graphene and graphene-like two-dimensional (2D) materials. The review ends with a prospect on the challenges and opportunities of non-covalent interaction-based molecular electronics in the near future.
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A single-molecule field-effect transistor (FET) is the key building block of future electronic circuits. At the same time, a single-molecule FET is also a unique platform for studying physical mechanisms...
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The thermoelectric properties of parallel arrays of organic molecules on a surface offer the potential for large-area, flexible, solution processed, energy harvesting thin-films, whose room-temperature transport properties are controlled by quantum interference (QI). Recently, it has been demonstrated that constructive QI (CQI) can be translated from single molecules to self-assembled monolayers (SAMs), boosting both electrical conductivities and Seebeck coefficients. However, these CQI-enhanced systems are limited by rigid coupling of the component molecules to metallic electrodes, preventing the introduction of additional layers which would be advantageous for their further development. These rigid couplings also limit our ability to suppress the transport of phonons through these systems, which could act to boost their thermoelectric output, without comprising on their impressive electronic features. Here, through a combined experimental and theoretical study, we show that cross-plane thermoelectricity in SAMs can be enhanced by incorporating extra molecular layers. We utilize a bottom-up approach to assemble multi-component thin-films that combine a rigid, highly conductive ‘sticky’-linker, formed from alkynyl-functionalised anthracenes, and a ‘slippery’-linker consisting of a functionalized metalloporphyrin. Starting from an anthracene-based SAM, we demonstrate that subsequent addition of either a porphyrin layer or a graphene layer increases the Seebeck coefficient, and addition of both porphyrin and graphene leads to a further boost in their Seebeck coefficients. This demonstration of Seebeck-enhanced multi-component SAMs is the first of its kind and presents a new strategy towards the design of thin-film thermoelectric materials.
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Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.
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Quantum interference effects (QI) are of interest in nano-scale devices based on molecular tunneling junctions because they can affect conductance exponentially through minor structural changes. However, their utilization requires the prediction and deterministic control over the position and magnitude of QI features, which remains a significant challenge. In this context, we designed and synthesized three benzodithiophenes based molecular wires; one linearly-conjugated, one cross-conjugated and one cross-conjugated quinone. Using eutectic Ga-In (EGaIn) and CP-AFM, we compared them to a well-known anthraquinone in molecular junctions comprising self-assembled monolayers. By combining density functional theory and transition voltage spectroscopy, we show that the presence of an interference feature and its position can be controlled independently by manipulating bond topology and electronegativity. This is the first study to separate these two parameters experimentally, demonstrating that the conductance of a tunneling junction depends on the position and depth of a QI feature, both of which can be controlled
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This overview will give a glimpse into chemical design principles for gating quantum interference (QI) effects in molecular-scale devices. Direct observation of room temperature QI in single-molecule junctions has stimulated growing interest in fabrication of tailor-made molecular electronic devices. Herein, we outline a new conceptual advance in the scientific understanding and technological know-how necessary to control QI effects in single molecules by chemical modification. We start by discussing QI from a chemical viewpoint and then describe a new magic ratio rule (MRR), which captures a minimal description of connectivity-driven charge transport and provides a useful starting point for chemists to design appropriate molecules for molecular electronics with desired functions. The MRR predicts conductance ratios, which are solely determined by QI within the core of polycyclic aromatic hydrocarbons (PAHs). The manifestations of QI and related quantum circuit rules for materials discovery are direct consequences of the key concepts of weak coupling, locality, connectivity, mid-gap transport and phase coherence in single-molecule junctions.
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Gate-tunable two-dimensional (2D) materials-based quantum capacitors (QCs) and van der Waals heterostructures involve tuning transport or optoelectronic characteristics by the field effect. Recent studies have attributed the observed gate-tunable characteristics to the change of the Fermi level in the first 2D layer adjacent to the dielectrics, while the penetration of the field effect through the one-molecule-thick material is often ignored or over-simplified. Here, we present a multiscale theoretical approach that combines first-principles electronic structure calculations and the Poisson-Boltzmann equation methods to model penetration of the field effect through graphene in a metal-oxide-graphene-semiconductor (MOGS) QC, including quantifying the degree of "transparency" for graphene two-dimensional electron gas (2DEG) to an electric displacement field. We find that the space charge density in the semiconductor layer can be modulated by gating in a nonlinear manner, forming an accumulation or inversion layer at the semiconductor/graphene interface. The degree of transparency is determined by the combined effect of graphene quantum capacitance and the semiconductor capacitance, which allows us to predict the ranking for a variety of monolayer 2D materials according to their transparency to an electric displacement field as follows: graphene > silicene > germanene > WS2 > WTe2 > WSe2 > MoS2 > phosphorene > MoSe2 > MoTe2, when the majority carrier is electron. Our findings reveal a general picture of operation modes and design rules for the 2D-materials-based QCs.
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Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, dangling-bond-free lattice and is weakly bound to neighbouring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate atomic layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics.
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Through molecular engineering, single diarylethenes were covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. Our experimental and theoretical studies of these junctions consistently show and interpret reversible conductance photoswitching at room temperature and stochastic switching between different conductive states at low temperature at a single-molecule level. We demonstrate a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy (on/off ratio of ~100), stability (over a year), and reproducibility (46 devices with more than 100 cycles for photoswitching and ~105 to 106 cycles for stochastic switching).
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Creating functional electrical circuits using individual or ensemble molecules, often termed as "molecular-scale electronics", not only meets the increasing technical demands of the miniaturization of traditional Si-based electronic devices, but also provides an ideal window of exploring the intrinsic properties of materials at the molecular level. This Review covers the major advances with the most general applicability and emphasizes new insights into the development of efficient platform methodologies for building reliable molecular electronic devices with desired functionalities through the combination of programmed bottom-up self-assembly and sophisticated top-down device fabrication. First, we summarize a number of different approaches of forming molecular-scale junctions and discuss various experimental techniques for examining these nanoscale circuits in details. We then give a full introduction of characterization techniques and theoretical simulations for molecular electronics. Third, we highlight the major contributions and new concepts of integrating molecular functionalities into electrical circuits. Finally, we provide a critical discussion of limitations and main challenges that still exist for the development of molecular electronics. These analyses should be valuable for deeply understanding charge transport through molecular junctions, the device fabrication process, and the roadmap for future practical molecular electronics.
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Molecular electronics aims to construct functional molecular devices at the single-molecule scale. One of the major challenges is to construct a single-molecule junction and to further manipulate the charge transport through the molecular junction. Break junction techniques, including STM break junctions and mechanically controllable break junctions are considered as testbed to investigate and control the charge transport on a single-molecule scale. Moreover, additional electrochemical gating provides a unique opportunity to manipulate the energy alignment and molecular redox processes for a single-molecule junction. In this review, we start from the technical aspects of the break junction technique, then discuss the molecular structure-conductance correlation derived from break junction studies, and, finally, emphasize electrochemical gating as a promising method for the functional molecular devices.
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This tutorial outlines the basic theoretical concepts and tools which underpin the fundamentals of phase-coherent electron transport through single molecules. The key quantity of interest is the transmission coefficient T(E), which yields the electrical conductance, current-voltage relations, the thermopower S and the thermoelectric figure of merit ZT of single-molecule devices. Since T(E) is strongly affected by quantum interference (QI), three manifestations of QI in single-molecules are discussed, namely Mach-Zehnder interferometry, Breit-Wigner resonances and Fano resonances. A simple MATLAB code is provided, which allows the novice reader to explore QI in multi-branched structures described by a tight-binding (Hückel) Hamiltonian. More generally, the strengths and limitations of materials-specific transport modelling based on density functional theory are discussed.
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We have developed an efficient simulation tool 'GOLLUM' for the computation of electrical, spin and thermal transport characteristics of complex nanostructures. The new multi-scale, multi-terminal tool addresses a number of new challenges and functionalities that have emerged in nanoscale-scale transport over the past few years. To illustrate the flexibility and functionality of GOLLUM, we present a range of demonstrator calculations encompassing charge, spin and thermal transport, corrections to density functional theory such as local density approximation +U (LDA+U) and spectral adjustments, transport in the presence of non-collinear magnetism, the quantum Hall effect, Kondo and Coulomb blockade effects, finite-voltage transport, multi-terminal transport, quantum pumps, superconducting nanostructures, environmental effects, and pulling curves and conductance histograms for mechanically-controlled break-junction experiments.
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The idea of using individual molecules as active electronic components provided the impetus to develop a variety of experimental platforms to probe their electronic transport properties. Among these, single-molecule junctions in a metal-molecule-metal motif have contributed significantly to our fundamental understanding of the principles required to realize molecular-scale electronic components from resistive wires to reversible switches. The success of these techniques and the growing interest of other disciplines in single-molecule-level characterization are prompting new approaches to investigate metal-molecule-metal junctions with multiple probes. Going beyond electronic transport characterization, these new studies are highlighting both the fundamental and applied aspects of mechanical, optical and thermoelectric properties at the atomic and molecular scales. Furthermore, experimental demonstrations of quantum interference and manipulation of electronic and nuclear spins in single-molecule circuits are heralding new device concepts with no classical analogues. In this Review, we present the emerging methods being used to interrogate multiple properties in single molecule-based devices, detail how these measurements have advanced our understanding of the structure-function relationships in molecular junctions, and discuss the potential for future research and applications.
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Understanding charge transport of single molecules or a small collection of molecules sandwiched between electrodes is of fundamental importance for molecular electronics. This requires the fabrication of reliable devices, which depend on several factors including the testbed architectures used, the molecule number and defect density being tested, and the nature of the molecule-electrode interface. On the basis of significant progresses achieved in both experiments and theory over the past decade, in this tutorial review, we focus on new insights into the influence of the nature of the molecule-electrode interface, the most critical issue hindering the development of reliable devices, on the conducting properties of molecules. We summarize the strategies developed for controlling the interfacial properties and how the coupling strength between the molecules and the electrodes modulates the device properties. These analyses should be valuable for deeply understanding the relationship between the contact interface and the charge transport mechanism, which is of crucial importance for the development of molecular electronics, organic electronics, nanoelectronics, and other interface-related optoelectronic devices.
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Using a first principles approach, we study the electron transport properties of two molecules of length 2.5 nm, which are the building blocks for a new class of molecular wires containing fluorenone units. We show that the presence of side groups attached to these units leads to Fano resonances close to the Fermi energy. As a consequence electron transport through the molecule can be controlled either by chemically modifying the side group, or by changing the conformation of the side group. This sensitivity, which is not present in Breit-Wigner resonances, opens up new possibilities for novel single-molecule sensors.
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Graphene has attracted considerable interest for future electronics, but the absence of a bandgap limits its direct applicability in transistors and logic devices. Recently, other layered materials such as molybdenum disulphide (MoS(2)) have been investigated to address this challenge. Here, we report the vertical integration of multi-heterostructures of layered materials for the fabrication of a new generation of vertical field-effect transistors (VFETs) with a room temperature on-off ratio > 10(3) and a high current density of up to 5,000 A cm(-2). An n-channel VFET is created by sandwiching few-layer MoS(2) as the semiconducting channel between a monolayer graphene sheet and a metal thin film. This approach offers a general strategy for the vertical integration of p- and n-channel transistors for high-performance logic applications. As an example, we demonstrate a complementary inverter with a larger-than-unity voltage gain by vertically stacking graphene, Bi(2)Sr(2)Co(2)O(8) (p-channel), graphene, MoS(2) (n-channel) and a metal thin film in sequence. The ability to simultaneously achieve a high on-off ratio, a high current density and a logic function in such vertically stacked multi-heterostructures can open up possibilities for three-dimensional integration in future electronics.
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We detailed a facile detection technique to optically characterize graphene growth and domains directly on growth substrates through a simple thermal annealing process. It was found that thermal annealing transformed the naked Cu to Cu oxides while keeping graphene and graphene-covered Cu intact. This increases the interference color contrast between Cu oxides and Cu, thus making graphene easily visible under an optical microscope. By using this simple method, we studied the factors that affect graphene nucleation and growth and achieved graphene domains with the domain size as large as ~100 μm. The concept of chemically making graphene visible is universal, as demonstrated by the fact that a solution process based on selective H(2)O(2) oxidation has been developed to achieve the similar results in a shorter time. These techniques should be valuable for studies towards elucidating the parameters that control the grains, boundaries, structures and properties of graphene.
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According to Kirchhoff's circuit laws, the net conductance of two parallel components in an electronic circuit is the sum of the individual conductances. However, when the circuit dimensions are comparable to the electronic phase coherence length, quantum interference effects play a critical role, as exemplified by the Aharonov-Bohm effect in metal rings. At the molecular scale, interference effects dramatically reduce the electron transfer rate through a meta-connected benzene ring when compared with a para-connected benzene ring. For longer conjugated and cross-conjugated molecules, destructive interference effects have been observed in the tunnelling conductance through molecular junctions. Here, we investigate the conductance superposition law for parallel components in single-molecule circuits, particularly the role of interference. We synthesize a series of molecular systems that contain either one backbone or two backbones in parallel, bonded together cofacially by a common linker on each end. Single-molecule conductance measurements and transport calculations based on density functional theory show that the conductance of a double-backbone molecular junction can be more than twice that of a single-backbone junction, providing clear evidence for constructive interference.
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As the dimensions of a conductor approach the nanoscale, quantum effects begin to dominate, and it becomes possible to control the conductance through direct manipulation of the electron wavefunction. Such control has been demonstrated in various mesoscopic devices at cryogenic temperatures, but it has proved to be difficult to exert control over the wavefunction at higher temperatures. Molecules have typical energy level spacings (∼eV) that are much larger than the thermal energy at 300 K (∼25 meV), and are therefore natural candidates for such experiments. Previously, phenomena such as giant magnetoresistance, Kondo effects and conductance switching have been observed in single molecules, and theorists have predicted that it should also be possible to observe quantum interference in molecular conductors, but until now all the evidence for such behaviour has been indirect. Here, we report the observation of destructive quantum interference in charge transport through two-terminal molecular junctions at room temperature. We studied five different rigid π-conjugated molecular wires, all of which form self-assembled monolayers on a gold surface, and find that the degree of interference can be controlled by simple chemical modifications of the molecular wire.
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We analyze quantum interference and decoherence effects in single-molecule junctions both experimentally and theoretically by means of the mechanically controlled break junction technique and density-functional theory. We consider the case where interference is provided by overlapping quasi-degenerate states. Decoherence mechanisms arising from the electronic-vibrational coupling strongly affect the electrical current flowing through a single-molecule contact and can be controlled by temperature variation. Our findings underline the all-important relevance of vibrations for understanding charge transport through molecular junctions.
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Using first-principles calculations we analyze the electronic transport properties of a recently proposed anthraquinone based electrochemical switch. Robust conductance on/off ratios of several orders of magnitude are observed due to destructive quantum interference present in the anthraquinone, but absent in the hydroquinone molecular bridge. A simple explanation of the interference effect is achieved by transforming the frontier molecular orbitals into localized molecular orbitals thereby obtaining a minimal tight-binding model describing the transport in the relevant energy range in terms of hopping via the localized orbitals. The topology of the tight-binding model, which is dictated by the symmetries of the molecular orbitals, determines the amount of quantum interference.
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The ultimate aim of molecular electronics is to understand and master single-molecule devices. Based on the latest results on electron transport in single molecules in solid-state devices, we focus here on new insights into the influence of metal electrodes on the energy spectrum of the molecule, and on how the electron transport properties of the molecule depend on the strength of the electronic coupling between it and the electrodes. A variety of phenomena are observed depending on whether this coupling is weak, intermediate or strong.
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Tuning the transport properties of molecular junctions by chemically modifying the molecular structure is one of the key challenges for advancing the field of molecular electronics. In the present contribution, we investigate current-voltage characteristics of differently linked metal-molecule-metal systems that comprise either a single molecule or a molecular assembly. This is achieved by employing density functional theory in conjunction with a Green's function approach. We show that the conductance of a molecular system with a specific anchoring group is fundamentally different depending on whether a single molecule or a continuous monolayer forms the junction. This is a consequence of collective electrostatic effects that arise from dipolar elements contained in the monolayer and from interfacial charge rearrangements. As a consequence of these collective effects, the "ideal" choice for an anchoring group is clearly different for monolayer and single molecule devices. A particularly striking effect is observed for pyridine-docked systems. These are subject to Fermi-level pinning at high molecular packing densities, causing an abrupt increase of the junction current already at small voltages.
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If quantum interference patterns in the hearts of polycyclic aromatic hydrocarbons (PAHs) could be isolated and manipulated, then a significant step towards realizing the potential of single-molecule electronics would be achieved. Here we demonstrate experimentally and theoretically that a simple, parameter-free, analytic theory of interference patterns evaluated at the mid-point of the HOMO-LUMO gap (referred to as M-functions) correctly predicts conductance ratios of molecules with pyrene, naphthalene, anthracene, anthanthrene or azulene hearts. M-functions provide new design strategies for identifying molecules with phase-coherent logic functions and enhancing the sensitivity of molecular-scale interferometers.
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Experiments using a mechanically-controlled break junction and calculations based on density functional theory demonstrate a new magic ratio rule (MRR), which captures the contribution of connectivity to the electrical conductance of graphene-like aromatic molecules. When one electrode is connected to a site i and the other is connected to a site i' of a particular molecule, we assign the molecule a "magic integer" Mii'. Two molecules with the same aromatic core, but different pairs of electrode connection sites (i,i' and j,j' respectively) possess different magic integers Mii' and Mjj'. Based on connectivity alone, we predict that when the coupling to electrodes is weak and the Fermi energy of the electrodes lies close to the centre of the HOMO-LUMO gap, the ratio of their conductances is equal to (Mii'/Mjj')2. The MRR is exact for a tight binding representation of a molecule and a qualitative guide for real molecules.
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We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (-CS2(-)) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8.
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The conductance superposition law of molecules connected in parallel between the two electrodes of a tunnel junction is discussed. For molecules directly adsorbed between the electrodes, a linear superposition law is found with corrections coming from the interaction between the molecules through the surface of the electrodes. For molecules connected to the electrodes via molecular wires (the molecular connection nodes being located inside the tunnel junction), a quadratic law is found. These two superposition regimes are illustrated on realistic conjugated oligomer molecular circuits.
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Molecular electronics: Charge transport through a single benzene ring is studied using the mechanically controlled break-junction technique. The low-bias conductance for a meta-coupled benzene ring is more than an order of magnitude smaller than that of a para-coupled benzene ring. This difference can be explained by a destructive quantum interference effects in the meta-coupled benzene.
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An aliphatic quaternary ammonium salt which has a methoxyethyl group on the nitrogen atom formed an ionic liquid (room temperature molten salt) when combined with the tetrafluoroborate (BF4−) and bis(trifluoromethylsulfonyl)imide [TFSI; (CF3SO2)2N−] anions. The limiting oxidation and reduction potentials, specific conductivity, and some other physicochemical properties of the novel ionic liquids, N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF4) and DEME-TFSI have been evaluated and compared with those of 1-ethyl-3-methylimidazolium tetrafluoroborate. DEME-BF4 is a practically useful ionic liquid for electrochemical capacitors as it has a quite wide potential window (6.0 V) and high ionic conductivity (4.8 mS cm−1 at 25 °C). We prepared an electric double layer capacitor (EDLC) composed of a pair of activated carbon electrodes and DEME-BF4 as the electrolyte. This EDLC (working voltage ∼2.5 V) has both, a higher capacity above room temperature and a better charge–discharge cycle durability at 100 °C when compared to a conventional EDLC using an organic liquid electrolyte such as a tetraethylammonium tetrafluoroborate in propylene carbonate.
Article
The charge transport characteristics of 11 tailor-made dithiol-terminated oligo(phenylene-ethynylene) (OPE)-type molecules attached to two gold electrodes were studied at a solid/liquid interface in a combined approach using an STM break junction (STM-BJ) and a mechanically controlled break junction (MCBJ) setup. We designed and characterized 11 structurally distinct dithiol-terminated OPE-type molecules with varied length and HOMO/LUMO energy. Increase of the molecular length and/or of the HOMO-LUMO gap leads to a decrease of the single-junction conductance of the linearly conjugate acenes. The experimental data and simulations suggest a nonresonant tunneling mechanism involving hole transport through the molecular HOMO, with a decay constant β = 3.4 ± 0.1 nm(-1) and a contact resistance R(c) = 40 kΩ per Au-S bond. The introduction of a cross-conjugated anthraquinone or a dihydroanthracene central unit results in lower conductance values, which are attributed to a destructive quantum interference phenomenon for the former and a broken π-conjugation for the latter. The statistical analysis of conductance-distance and current-voltage traces revealed details of evolution and breaking of molecular junctions. In particular, we explored the effect of stretching rate and junction stability. We compare our experimental results with DFT calculations using the ab initio code SMEAGOL and discuss how the structure of the molecular wires affects the conductance values.
Article
Electronic factors in molecules such as quantum interference and cross-conjugation can lead to dramatic modulation and suppression of conductance in single-molecule junctions. Probing such effects at the single-molecule level requires simultaneous measurements of independent junction properties, as conductance alone cannot provide conclusive evidence of junction formation for molecules with low conductivity. Here, we compare the mechanics of the conducting para-terminated 4,4'-di(methylthio)stilbene and moderately conducting 1,2-bis(4-(methylthio)phenyl)ethane to that of insulating meta-terminated 3,3'-di(methylthio)stilbene single-molecule junctions. We simultaneously measure force and conductance across single-molecule junctions and use force signatures to obtain independent evidence of junction formation and rupture in the meta-linked cross-conjugated molecule even when no clear low-bias conductance is measured. By separately quantifying conductance and mechanics, we identify the formation of atypical 3,3'-di(methylthio)stilbene molecular junctions that are mechanically stable but electronically decoupled. While theoretical studies have envisaged many plausible systems where quantum interference might be observed, our experiments provide the first direct quantitative study of the interplay between contact mechanics and the distinctively quantum mechanical nature of electronic transport in single-molecule junctions.
Article
Employing a scanning tunneling microscopy based beak junction technique and mechanically controlled break junction experiments, we investigated tolane (diphenylacetylene)-type single molecular junctions having four different anchoring groups (SH, pyridyl (PY), NH(2), and CN) at a solid/liquid interface. The combination of current-distance and current-voltage measurements and their quantitative statistical analysis revealed the following sequence for junction formation probability and stability: PY > SH > NH(2) > CN. For all single molecular junctions investigated, we observed the evolution through multiple junction configurations, with a particularly well-defined binding geometry for PY. The comparison of density functional theory type model calculations and molecular dynamics simulations with the experimental results revealed structure and mechanistic details of the evolution of the different types of (single) molecular junctions upon stretching quantitatively.
Article
Superconductivity at interfaces has been investigated since the first demonstration of electric-field-tunable superconductivity in ultrathin films in 1960(1). So far, research on interface superconductivity has focused on materials that are known to be superconductors in bulk. Here, we show that electrostatic carrier doping can induce superconductivity in KTaO(3), a material in which superconductivity has not been observed before. Taking advantage of the large capacitance of the self-organized electric double layer that forms at the interface between an ionic liquid and KTaO(3) (ref. 12), we achieve a charge carrier density that is an order of magnitude larger than the density that can be achieved with conventional chemical doping. Superconductivity emerges in KTaO(3) at 50 mK for two-dimensional carrier densities in the range 2.3 × 10(14) to 3.7 × 10(14) cm(-2). The present result clearly shows that electrostatic carrier doping can lead to new states of matter at nanoscale interfaces.
Article
Quantum interference (QI) of electron pathways has recently attracted increased interest as an enabling tool for single-molecule electronic devices. Although various molecular systems have been shown to exhibit QI effects and a number of methods have been proposed for its analysis, simple guidelines linking the molecular structure to QI effects in the phase-coherent transport regime have until now been lacking. In the present work we demonstrate that QI in aromatic molecules is intimately related to the topology of the molecule's π system and establish a simple graphical scheme to predict the existence of QI-induced transmission antiresonances. The generality of the scheme, which is exact for a certain class of tight-binding models, is proved by a comparison to first-principles transport calculations for 10 different configurations of anthraquinone as well as a set of cross-conjugated molecular wires.
Article
Quantum interference in coherent transport through single molecular rings may provide a mechanism to control the current in molecular electronics. We investigate its applicability, using a single-particle Green function method combined with ab initio electronic structure calculations. We find that the quantum interference effect (QIE) is strongly dependent on the interaction between molecular pi-states and contact sigma-states. It is masked by sigma tunneling in small molecular rings with Au leads, such as benzene, due to strong pi-sigma hybridization, while it is preserved in large rings, such as [18]annulene, which then could be used to realize quantum interference effect (QIE) transistors.
Article
An overview is given of the preparation, formation, structure, and applications of self-assembled monolayers (SAMs) formed from alkanethiols (and derivatives of alkanethiols) on gold, silver, copper, palladium, platinum, mercury, and alloys of these metals. Emphasis is on advances made in this area over the past five years (1999-2004). First, the structure and mechanism of formation of SAMs formed by adsorption of n-alkanethiols on metals are described. Following this, the applications of SAMs where they act as nanostructures themselves, enable other nanosystems, interact with biological nanostructures, and form patterns on surfaces with critical dimensions below 100 nm are outlined. Furthermore, an attempt is made to outline what is not understood about these SAMs and which of their properties are not yet controlled. Finally, some of the important opportunities that still remain for future progress in research involving SAMs are sketched.
  • K S Novoselov
  • A Mishchenko
  • A Carvalho
  • Castro Neto
Novoselov, K.S., Mishchenko, A., Carvalho, A., and Castro Neto, A.H. (2016). 2D materials and van der Waals heterostructures. Science 353, aac9439.