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Optoelectronic Switching of a Carbon Nanotube Chiral Junction Imaged with Nanometer Spatial Resolution
Abstract
Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting-metallic junction. We image topography, laser absorption at 532 nm, and measure I-V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting-metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated due to the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.
... In addition, strong electric fields emerge at the tip/sample junction due to the mandatory voltage which must be applied to either the tip or to the sample in STM experiments. 35 These requirements pose significant challenges with respect to lead halide perovskites where the ABX 3 crystal structure (A-site: organic or inorganic cation[s], B-site: lead, C-site: halides, eg, Cl, Br, I) also mirrors the weak point in these materials: the soft, dynamic and ionic bonding character of the crystal lattice. 36,37 In this critical review, we highlight the recent progress in applying STM-based techniques to organic/inorganic and all-inorganic lead halide perovskites, both in form of single crystals and thin films. ...
... These are the result of the short tip-sample distance on the order of nanometers required for quantum tunneling. 35 It has previously been shown that activation energies below 1 eV are sufficient to displace ions in the perovskite material and distort the perovskite lattice where the migration activation energies are the lowest for the halides followed by the A-site cations. [55][56][57][58] This results in various, and most of the time unpredictable and unforeseen, surface and bulk structures depending on the strength and polarity of the electric field. ...
... Here, STM techniques such as single-molecule absorption by STM (SMA-STM) or novel photocurrent measurement protocols using STM have been developed which have been already applied successfully to other semiconductor systems. 34,35,[69][70][71] The SMA-STM technique makes use of a unique light coupling geometry which avoids excessive heating of the tipsample junction thus allows precise investigation of the changes in the DOS under illumination. ...
Since the introduction of lead halide perovskites, tremendous progress has been made regarding their stability, reproducibility and durability. However, one of the issues that remains is related to the interfacial atomic structure arrangement and structure‐property relationship under optical and electrical stimuli. In this critical review, we highlight the recent progress using scanning tunneling microscopy (STM) to understand the nanoscale properties and dynamic processes occurring in these halide perovskite materials. STM is known to be a very challenging technique, which is reflected by the low number of relevant publications in the last years. These initial reports mirror the unique potential of STM to give Ångstrom‐scale insight into the (opto)‐electronic properties, morphology and underlying electronic structure and provide a path toward harnessing the full potential of these materials. However, care must be taken to understand the effects of the perturbations caused by STM and tailor the measurement conditions accordingly.
... The resulting SMA-STM image is shown in Fig. 6.2d. As we have established in the past, [354][355][356][357]359,361 darker regions in such images correspond to locations of larger tunneling current modulation, which are effectively locations of higher light absorption, relative to the substrate absorption. Fig. 6.2d shows that the tunneling current modulation level is different for each nanoisland. ...
... Such a process would block tunneling through the previously empty states, which would be manifested in a lower tunneling current under photoexcitation. 356 A nanoisland with an LSP resonance overlapping with the 700 nm excitation wavelength will absorb light more strongly as compared to the PtAu substrate, which itself has broad absorption across the near-infrared (NIR) region ( Fig. 6.1) Such a nanoisland would exhibit a negative SMA-STM signal. ...
... 6.1, an incident laser illuminates the sample from the rear in a total-internal-reflection (TIR) Kretschmann configuration achieved by machining a wedge into a sapphire substrate. This rear illumination allows for the generation of LSPs at the Au surface and minimizes thermal perturbation at the STM tip-sample junction.355,356,360,361 The sample surface is scanned by the STM tip in the presence of the laser beam. ...
The Jain lab employs a topotactic method called cation exchange to produce semiconductor nanocrystals (NCs) in novel morphologies, compositions, and crystallographic phases. My dissertation research focuses on the understanding of the physical properties and phase transitions of these new nanomaterials prepared by cation exchange. In Chapter 1, I describe the countless possibilities of the exploration of physicochemical properties and applications of molecularly precise semiconductor nanoclusters, a class of materials that we were able to expand with the help of cation exchange. In Chapter 2, I discuss how ultrasmall copper selenide (Cu2-xSe) NCs prepared by cation exchange of cadmium selenide NCs exhibit a disordered cationic sub-lattice under ambient conditions. This behavior is quite unlike larger NCs or the bulk, suggesting an interesting effect of crystallite size and strain on the stability of super-ionic phases. In Chapter 3, I describe my investigations of Li-doping of Cu2-xSe NCs and how this doping influences the crystal structure and consequently the phase transition behavior. A close-to-ambient-temperature transition from the non-superionic to superionic phase transition also appears to be present in the final lithium selenide (Li2Se) NCs formed from this doping reaction. In Chapter 4, I explain on the basis of optical spectra measurements and density functional theory (DFT) calculations how HgSe NCs, prepared using cation exchange in a novel wurtzite phase, differ from their natural zinc-blende counterparts. The latter is a semi-metal, whereas the newer phase obtained from cation exchange is found to have an inverted band structure along with a finite band-gap, making it a potential 3D topological insulator. In Chapter 5, I extend the understanding of ion exchange reactions to an “anion exchange” process in zinc oxide (ZnO) NCs. As a detour from the central thesis of my dissertation, in Chapter 6, I present my work on electrodynamic simulations of optical properties of nanostructures, which helped demonstrate that localized surface plasmons can be imaged in real space with nanometer resolution using a scanning tunneling microscope (STM) coupled to a laser.
... 14 Longitudinal localization in a single CNT has been studied extensively and visualized at subnanometer resolution. 12,15,16 One of the most promising applications of CNTs is in CNT FETs, as demonstrated previously. 5,17 In this application, a voltage (transverse electric field) is applied across the CNT to change its conductivity to turn on and off the current. ...
... The direct imaging of a localized SMA-STM signal along the back edge of a defectfree segment of the CNT is different from resonant excitation of an isolated CNT, where the SMA-STM signal is uniform across the entire CNT, or from single-point scanning tunneling spectroscopy (STS I−V curve). 15,16 Based on the oblong shape of the QD in the topography image in Figure 1a, it has been rotated by ∼120°counterclockwise in the plane of the surface in Figure 1c in addition to being translated. This effect has been described previously and can be used to obtain multiple 2D projections of excited states of a QD with sub-nanometer spatial resolution. ...
... A weak signal that fluctuates across the CNT is seen at 1.3 and 1.5 V. In contrast, a previous study of isolated CNTs on a PtAu surface 16 showed uniform signal changes across the CNTs, attributed to a Stark effect-induced spectral shift in the absorption spectrum while the CNT was probed at a fixed wavelength. ...
Efficient heat dissipation and large gate capacitance have made carbon nanotube field-effect transistors (CNT FETs) devices of interest for over 20 years. The mechanism of CNT FETs involves localization of the electronic structure due to a transverse electric field, yet little is known about the localization effect, nor has the electronic polarization been visualized directly. Here, we co-deposit PbS quantum dots (QD) with CNTs and optically excite the QD so its excited state dipolar field biases the local environment of a CNT. Using single molecule absorption scanning tunneling microscopy, we show that the electronic states of the CNT become transversely localized. By nudging QDs to different distances from the CNT, the magnitude of the localization can be controlled. Different bias voltages probe the degree of localization in different CNT excited states. A simple tight binding model for the CNT in an electrostatic field provides a semi-quantitative model for the observed behavior.
... This rear illumination allows for the generation of LSPs at the Au surface and minimizes thermal perturbation at the STM tip−sample junction. 50,51,55,56 The sample surface is scanned by the STM tip in the presence of the laser beam. To enhance detection sensitivity, the laser is amplitude-modulated. ...
... The resulting SMA-STM image is shown in Figure 2d. As we have established in the past, [49][50][51][52]54,56 darker regions in such images correspond to locations of larger tunneling current modulation, which are effectively locations of higher light absorption, relative to the substrate absorption. Figure 2d shows that the tunneling current modulation level is different for each nanoisland. ...
... Such a process would block tunneling through the previously empty states, which would be manifested in a lower tunneling current under photoexcitation. 51 A nanoisland with a LSP resonance overlapping with the 700 nm excitation wavelength will absorb light more strongly as compared to the PtAu substrate, which itself has broad absorption across the nearinfrared (NIR) region ( Figure S1) Such a nanoisland would exhibit a negative SMA-STM signal. This is the case for nanoisland 1. Conversely, a nanoisland may have a LSP resonance completely offset from the excitation wavelength. ...
An optically modulated scanning tunneling microscopy technique developed for measurement of single-molecule optical absorption is used here to image the light absorption by individual Au nanoislands and Au nanostructures. The technique is shown to spatially map, with nanometer resolution, localized surface plasmons (LSPs) excited within the nanoislands. Electrodynamic simulations demonstrate the correspondence of the measured images to plasmonic near-field intensity maps. The optical STM imaging technique captures the wavelength, polarization, and geometry dependence of the LSP resonances and their corresponding near-fields. Thus, we introduce a tool for real-space, nanometer-scale visualization of optical energy absorption, transport, and dissipation in complex plasmonic nanostructures.
... Single-molecule absorption by STM (SMA-STM) provides a mean to study local changes induced by optical illumination at the nanoscale (schematic in Figure 3e). [52,68,69] Under modulated optical illumination, the local absorption can be monitored across the sample interface simultaneously to the sample topography providing information about local variations in defect density (Figure 3g). It was further shown that the conduction band (CB) onset shifts concomitant with an increase in the LDOS population under visible illumination due to the creation of free carriers and new tunneling pathways for a mixed A-site mixed-halide composition. ...
Perovskite materials are promising contenders as the active layer in light‐harvesting and light‐emitting applications if their long‐term stability can be sufficiently increased. Chemical and structural engineering are shown to enhance long‐term stability, but the increased complexity of the material system also leads to inhomogeneous functional properties across various length scales. Thus, scanning probe and high‐resolution microscopy characterization techniques are needed to reveal the role of local defects and the results promise to act as the foundation for future device improvements. A look at the parameter space: technique‐specific sample penetration depth versus probe size highlights a gap in current methods. High spatial resolution combined with a deep penetration depth is not yet achievable. However, multimodal measurement technique may be the key to covering this parameter space. In this perspective, current advanced spectro‐microscopy methods which have been applied to perovskite materials are highlighted.
... Various studies have reported the use of chiral SWCNTs and MWCNTs for biosensing [326], electronics [327], photovoltaics [328] and optoelectronic applications, including the observation of optical tuning/switching of chiral semiconducting-metallic CNT junctions [329]. ...
Self-assembly offers unique possibilities for fabricating nanostructures, with different morphologies and properties, typically from vapor or liquid phase precursors. Molecular units, nanoparticles, biological molecules and other discrete elements can spontaneously organise or form via interactions at the nanoscale. Currently, nanoscale self-assembly finds applications in a wide variety of areas including carbon nanomaterials and semiconductor nanowires, semiconductor heterojunctions and superlattices, the deposition of quantum dots, drug delivery, such as mRNA-based vaccines, and modern integrated circuits and nanoelectronics, to name a few. Recent advancements in drug delivery, silicon nanoelectronics, lasers and nanotechnology in general, owing to nanoscale self-assembly, coupled with its versatility, simplicity and scalability, have highlighted its importance and potential for fabricating more complex nanostructures with advanced functionalities in the future. This review aims to provide readers with concise information about the basic concepts of nanoscale self-assembly, its applications to date, and future outlook. First, an overview of various self-assembly techniques such as vapour deposition, colloidal growth, molecular self-assembly and directed self-assembly/hybrid approaches are discussed. Applications in diverse fields involving specific examples of nanoscale self-assembly then highlight the state of the art and finally, the future outlook for nanoscale self-assembly and potential for more complex nanomaterial assemblies in the future as technological functionality increases.
... More recent reports of quantum dot excited-state imaging with SMA-STM further show its ability to probe the orientation-dependent imaging of quantum dot orbital structures (25). Alongside experiments with quantum dot systems, this technique has shown the ability to identify differences in excited-state electronic structure at chiral junctions in a single carbon nanotube (37). Figure 4 shows the voltage tuning of a junction between semiconducting and metallic regions in a carbon nanotube to a near metallic-metallic region, with resolution previously shown to be capable of imaging penetration across defects on the order of ∼0.9 nm (35). ...
At the intersection of spectroscopy and microscopy lie techniques that are capable of providing subnanometer imaging of excited states of individual molecules or nanoparticles. Such approaches are particularly important for imaging macromolecules or nanoparticles large enough to have a high probability of containing a defect. These inevitable defects often control properties and function despite an otherwise ideal structure. We discuss real-space imaging techniques such as using scanning tunneling microscopy tips to enhance optical measurements and electron energy-loss spectroscopy in a scanning transmission electron microscope, which is based on focused electron beams to obtain high-resolution spatial information on excited states. The outlook for these methods is bright, as they will provide critical information for the characterization and improvement of energy-switching, electron-switching, and energy-harvesting materials.
... 8 Highly excited S-and Plike states of quantum dots with defects have been imaged with sub-nanometer resolution 9 by using single-molecule laserabsorption detected by scanning tunneling microscopy (SMA-STM), which also has been applied to carbon nanotubes to study defects and exciton size. [9][10][11] In other studies, the STM tip has been successfully used to manipulate individual atoms, 12 small molecules, 13 graphene nanoribbons, 14 and carbon nanotubes 15 on a surface. Quantum dots have been manipulated into chains to allow energy transfer down the chain. ...
We previously demonstrated that we can image electronic excitations of quantum dots by single-molecule absorption scanning tunneling microscopy (SMA-STM). With this technique, a modulated laser beam periodically saturates an electronic transition of a single nanoparticle, and the resulting tunneling current modulation ΔI(x0, y0) maps out the SMA-STM image. In this paper, we first derive the basic theory to calculate ΔI(x0, y0) in the one-electron approximation. For near-resonant tunneling through an empty orbital “i” of the nanostructure, the SMA-STM signal is approximately proportional to the electron density φix0,y02 of the excited orbital in the tunneling region. Thus, the SMA-STM signal is approximated by an orbital density map (ODM) of the resonantly excited orbital at energy Ei. The situation is more complex for correlated electron motion, but either way a slice through the excited electronic state structure in the tunneling region is imaged. We then show experimentally that we can nudge quantum dots on the surface and roll them, thus imaging excited state electronic structure of a single quantum dot at different orientations. We use density functional theory to model ODMs at various orientations, for qualitative comparison with the SMA-STM experiment. The model demonstrates that our experimentally observed signal monitors excited states, localized by defects near the surface of an individual quantum dot. The sub-nanometer super-resolution imaging technique demonstrated here could become useful for mapping out the three-dimensional structure of excited states localized by defects within nanomaterials.
... An additional application of CNTs is as nano-probes. Here, carbon nano-probes can be exploited as atomic force microscopy [174,175] or scanning tunnelling microscope [176,177] tips. ...
Carbon has long been applied as an electrochemical sensing interface owing to its unique electrochemical properties. Moreover, recent advances in material design and synthesis particularly nano materials, has produced robust electrochemical sensing systems that display superior analytical performance. Carbon nanotubes (CNTs) are one of the most extensively studied nanostructures because of their unique properties. In terms of electroanalysis, the ability of CNTs to augment the electrochemical reactivity of important biomolecules and promote electron-transfer reactions of proteins is of particular interest. The remarkable sensitivity of CNTs to changes in surface conductivity due to the presence of adsorbates permits their application as highly sensitive nanoscale sensors. CNT-modified electrodes have also demonstrated their utility as anchors for biomolecules such as nucleic acids, and their ability to diminish surface fouling effects. Consequently, CNTs are highly attractive to researchers as a basis for many electrochemical sensors. Similarly, synthetic diamonds electrochemical properties, such as superior chemical inertness and biocompatibility, make it desirable for both (bio) chemical sensing and as the electrochemical interface for biological systems. This is highlighted by the recent development of multiple electrochemical diamond based biosensors and bio interfaces.
... After a tip change midway through scanning the largest dot (arrows in Figure 2a,b), the absorption signal decreases. As shown previously, 26,33 the absorption cross section is sensitive to the Stark effect from the tip's electric field, and can be taken out of resonance by a tip change. If the signal in Figure 2b indeed corresponds to energy transfer, it would be the absorption of the smallest dot that was partly shifted out of resonance. ...
Many processes of interest in quantum dots involve charge or energy transfer from one dot to another. Energy transfer in films of quantum dots as well as between linked quantum dots has been demonstrated by luminescence shift, and the ultrafast time-dependence of energy transfer processes has been resolved. Bandgap variation among dots (energy disorder) and dot separation are known to play an important role in how energy diffuses. Thus it would be very useful if energy transfer could be visualized directly on a dot-by-dot basis among small clusters or within films of quantum dots. To that effect, we report single molecule optical absorption detected by scanning tunneling microscopy (SMA-STM) to image energy pooling from donor into acceptor dots on a dot-by-dot basis. We show that we can manipulate groups of quantum dots by pruning away the dominant acceptor dot, and switching the energy transfer path to a new acceptor dot. Our experimental data agrees well with a simple Monte Carlo lattice model of energy transfer, similar to models in the literature, in which excitation energy is transferred preferentially from dots with a larger bandgap to dots with a smaller bandgap.
... With tunneling of electrons from occupied states (VB) or to an empty level (CB) of a semiconductor, the differential conductance (dI/dV ) spectrum provides the location of the band edges at the point of measurement. The band diagram in a range of heterostructures has also been constructed through this technique [7][8][9][10]. In a complex nanostructure, such as a junction in a nanorod, we earlier mapped the band edges along its length [4]. ...
Through scanning tunneling spectroscopy, we envisage imaging a heterostructure, namely a junction formed in a single nanorod. While the differential conductance spectrum provides location of conduction and valence band edges, dI/dV images record energy levels of materials. Such dI/dV images at different voltages allowed us to view p- and n-sections of heterojunction nanorods and more importantly the depletion region in such a junction that has a type-II band alignment. Viewing of selective sections in a heterojunction occurred due to band-bending in the junction and is correlated to the density of states spectrum of the individual semiconductors. The dI/dV images recorded at different voltages could be used to generate a band diagram of a pn junction.
... The dots are subject to a tunable electric field from the STM tip. 36 The well-studied quadratic Stark effect of QDs 38−42 allows different electronic states to be tuned into resonance at constant laser wavelengths of 660 or 532 nm. Laser modulation results in a small tunneling current modulation that detects the difference between groundstate and excited-state electronic density of states. ...
Electronically excited orbitals play a fundamental role in chemical reactivity and spectroscopy. In nanostructures, orbital shape is diagnostic of defects that control blinking, surface carrier dynamics and other important optoelectronic properties. We capture such nanometer resolution images of electronically excited PbS quantum dots by single molecule absorption scanning tunneling microscopy (SMA-STM). Dots with a bandgap of ~1 eV are deposited on a transparent gold surface and optically excited with red or green light to produce hot carriers. The STM tip-enhanced laser light produces a large excited state population, and the Stark effect allows transitions to be tuned into resonance by changing the sample voltage. Scanning the quantum dots under laser excitation, we were able to image electronic excitation to different angular momentum states depending on sample bias. The shapes differ from idealized S- or P-like orbitals due to imperfections of the quantum dots. Excitation of adjacent quantum dot pairs reveals orbital alignment, evidence for electronic coupling between dots. Electronic structure modeling of a small PbS quantum dot, when scaled for size, reveals Stark tuning and variation in the transition moment of different parity states, supporting the simple one-electron experimental interpretation in the hot carrier limit. The calculations highlight the sensitivity of orbital density to applied field, laser wavelength, and structural fluctuations of the quantum dot.
In a recent work, Nguyen et al. used trSMA-STM to probe electron-phonon dynamics within single carbon dots with nanometer resolution on a picosecond timescale. This promising technique opens up novel avenues to understand fluorescence quantum yield variations in carbon-dot ensembles.
Lead halide perovskites have emerged as promising absorber materials over the last decade to increase the efficiency of photovoltaics beyond its current limits. However, to further optimize the performance of perovskites more detailed studies need to be performed, which allow for the correlation of film morphology and local electronic properties at the nanoscale. Here, we present a scanning tunneling microscopy (STM) approach to probe the effect of an applied electric field of a methylammonium formamidinium lead triiodide perovskite thin film on the film response by current-voltage spectroscopy, current imaging tunneling spectroscopy, differential conductance mapping, and x-ray absorption spectroscopy by means of synchrotron x-ray STM. We find a strong correlation between the measurement conditions and the obtained current-voltage characteristics when imaging under opposite bias polarities. In particular, we find similarities to already observed poling effects for lead halide perovskites, which result in either a positively or negatively charged interface due to ion and vacancy migration. Our results provide insight into the influence of measurement conditions such as bias polarity on the performance assessment of perovskite thin films by STM.
Studying nanoscale photophysical processes is mandatory to fully understand the complex optoelectronic properties in semiconductor materials used in photovoltaics and light emitting diodes. In this perspective, we target specific scanning probe techniques, which combine scanning tunneling microscopy (STM) with optical methods to unravel the localized optoelectronic properties of semiconductors under realistic electric and optical fields, down to the nanoscale. Combining optical spectroscopy with STM yields a powerful platform that allows for simultaneous imaging of the surface morphology and the electronic structure down to the atomic level, a resolution that is otherwise not accessible due to the optical diffraction limit. Incident wavelengths spanning the electromagnetic spectrum from the terahertz region to X-rays have been coupled into the STM tip-sample junction to investigate the nanoscale properties of semiconductor materials, whereas the reverse process of luminescence can give insight on local recombination processes. Imagine the potential of a tool capable of detecting both localized absorption and spontaneous and stimulated emission processes of semiconductor materials at the nanoscale. The role of every atom, defect, or electronic interaction could be disentangled, tailored, or harnessed to its maximum capacity.
We prepared titanyl phthalocyanine (TiOPc) monolayers on sapphire prism supported Au{111} substrates through solution deposition and characterized their photo-responses using a custom built laser assisted scanning tunneling microscope under ambient conditions. Two types of lattice structures (hexagonal and rectangular) were observed and the distributions of photoinduced charges were measured under evanescently introduced 633 nm and 780 nm laser illumination. The distributions of photo-excited electrons in molecules in hexagonal lattices match theoretically calculated charge density changes in TiOPc molecules upon excitation. However, the photo-responses of TiOPc molecules in rectangular lattices are different than those predicted and TiOPc molecules in these arrangements may have lower excitation probabilities at 633 nm and 780 nm. Our results suggest that the photo carrier generation efficiency of TiOPc molecules is related to their packing arrangements in monolayers and local environments. The growth mechanisms of the monolayers and their connections to different polymorphs of TiOPc are worthy of further exploration so that the photo-generated electronic property changes that we observed at the molecular level can provide more accurate guidance to the application of TiOPc crystals and assemblies.
We measured photo-induced charge separation in isolated individual C60 tethered 2,5 dithienylpyrrole triad (C60 triad) molecules with submolecular resolution using a custom built laser assisted scanning tunneling microscope. Laser illumination was introduced evanescently into the tunneling junction through total internal reflection and the changes in tunneling current and electronic spectra caused by photoexcitation were measured and spatially resolved. Photo-induced charge separation was not detected for all C60 triad molecules, indicating that the conformations of the molecules may affect the excitation probability, lifetime, and/or charge distribution. Photo induced signal was not observed for dodecanethiol molecules in the surrounding matrix nor for control molecules without C60 moieties, as neither absorbs incident photons at this energy. This spectroscopic imaging technique has the potential to elucidate detailed photo-induced carrier dynamics, which are inaccessible via ensemble-scale (i.e., averaging) measurements, that can be used to direct the rational design and optimization of molecular p-n junctions and assemblies for energy harvesting.
Host-guest complexes comprising of cycloparaphenylene and carbon-based nanostructures have received enormous interest as a new class of materials in organic electronics. In the present study, the interactions of [10]cycloparaphenylene (CPP) and its derivatives with armchair carbon nanotube (CNT) are investigated by means of dispersion-corrected density functional theory. Different derivatives of cycloparaphenylene including nitrogen-doped, donor amino- and acceptor fluorine-substituted compounds viz., nN-[10]CPP, nNH2-[10]CPP and nF-[10]CPP, where n = 10, 20, 40 are modeled. The values of stabilization energies reveal that the complexes of CPP with CNTs are energetically stable. Among the complexes of various CPP derivatives, only those with fluorine substitutions are found to be stable. The energy decomposition analysis suggests that the complexes are stabilized mainly by the dispersion interactions between constituent units. The optoelectronic properties of the complexes are investigated using the time-dependent density functional theoretical (TD-DFT) method and the results indicate that the complexes of bare CPP with CNT absorb in the near-ultraviolet and visible regions, whereas those of fluorine-substituted CPP with CNT absorb in wider range starting from near-ultraviolet to near-infrared region of the electromagnetic spectrum. The frontier molecular orbital analysis reveals photoinduced charge transfer in tetra-fluorinated complex. All complexes exhibit narrow energy gap between the highest occupied and the lowest unoccupied molecular orbitals. The high values of light-harvesting efficiency obtained for fluorine-substituted complexes suggest their use in photovoltaic devices. The studies also show that there is an energy barrier for the piston type movement of CNT in the complexes for the ground state, but not for the excited states. The barrier for rotation of bare and fluorinated CPP over CNT suggests the potential of these complexes as components in molecular wheels and shuttles.
Nanometer-sized metal clusters are prime candidates for photoactivated catalysis, based on their unique tunable optical and electronic properties, combined with a large surface-to-volume ratio. Due to the very small optical cross sections of such nanoclusters, support-mediated plasmonic activation could potentially make activation more efficient. Our support is a semi-transparent gold film, optimized to work in a back-illumination geometry. It has a surface plasmon resonance excitable in the 510 to 540 nm wavelength range. Ptn clusters (size distribution peaked at n=46 atoms) have been deposited onto this support and investigated for the photoactivated catalytic performance in the oxidative decomposition of methylene blue. The Pt cluster catalytic activity under illumination exceeds that of the gold support by more than an order of magnitude per active surface area. To further investigate the underlying mechanism of plasmon-induced catalysis, the clusters have been imaged with optically-assisted scanning tunneling microscopy under illumination. The photoactivation of the Pt clusters via plasmonic excitation of the support and subsequential electronic excitation of the clusters can be imaged with nanometer resolution. The light-induced tunneling current on the clusters is enhanced relative to the gold film support.
With scanning tunneling spectroscopy (STS), we probed differential conductance (dI/dV) images of P3HT:PCBM bulk-heterojunctions (BHJs). Since the materials are seen energetically in dI/dV images, the imaging process provides opportunities to view the nano-domains of the components in the BHJ. The images were recorded at different voltages to bring out the interface region between the two materials as well. The density of states (DOS) spectra of pristine materials provided location of energy levels of the polymer and the fullerene in forming energy diagram with a type-II alignment from the view-point of charge carriers. DOS spectra recorded in the P3HT:PCBM heterojunction in addition yielded energies that deviated from those of the components indicating bending of energy levels at the interface region.
In this issue of ACS Nano, Nienhaus et al. report the optoelectronic properties of carbon nanotube chiral junctions with nanometer resolution in the presence of strong electric fields (∼1 V/nm). Here, we provide an overview of recent studies that combine scanning tunneling microscope (STM) and laser or microwave illumination. These techniques reveal nanoscale laser- or microwave-induced phenomena utilizing the intrinsic atomic resolution of the tunneling current, and do not require substantial modification of the STM itself. The merits of atomic-scale spatial resolution and chemical sensitivity of the laser or microwave spectroscopes make these techniques useful for nanoscale characterization.
Illumination is known to induce stress and morphology changes in opaque glasses. Amorphous silicon carbide (a-SiC) has a smaller bandgap than the crystal. Thus, we were able to excite with 532 nm light a 1 μm amorphous surface layer on a SiC crystal while recording time-lapse movies of glass surface dynamics by scanning tunneling microscopy
(STM).
Photoexcitation of the a-SiC surface layer through the transparent crystal avoids heating the STM tip. Up to 6 × 104 s, long movies of surface dynamics with 40 s time resolution and sub-nanometer spatial resolution were obtained. Clusters of ca. 3-5 glass forming units diameter are seen to cooperatively hop between two states at the surface.
Photoexcitation with green laser light recruits immobile clusters to hop, rather than increasing the rate at which already mobile clusters hop. No significant laser heating was observed. Thus, we favor an athermal mechanism whereby electronic excitation of a-SiC directly controls glassy
surface dynamics. This mechanism is supported by an exciton migration-relaxation-thermal diffusion model. Individual clusters take ∼1 h to populate states differently after the light intensity has changed. We believe the surrounding matrix rearranges slowly when it is stressed by a change in laser intensity, and clusters serve as a diagnostic. Such cluster hopping and matrix rearrangement could underlie the microscopic mechanism of photoinduced aging of opaque glasses.
where ~ Rn1n2 = n1~ a1 + n2~ a2 denotes the position of a carbon atom A and~ χ = ¡(~ a1+~ a2)=3 the position of the correspond- ing atom B in the same cell. The hexagonal lattice unit vec- tors are given by~ a1 = acc p 3 2 (1; p 3) and~ a 2 = acc p 3 2 (¡1; p 3), with acc = 1:42 ˚ A. The unit cell of straights CNs are specified by a chiral vec- tor ~ Ch = n~ a1+m~ a2 in the circumferential direction and a trans- lational vector ~ T = p~ a1 + q~ a2, along the longitudinal direc- tion. For zigzag CNs the translational symmetry along the y-direction is preserved and the coefficients CA and CB may be redefined in terms of a wave vector ky. One then gets the following set of coupled equations
We investigate the quantum transport through a single-walled carbon nanotube connected to leads in the presence of an external radiation field. We analyze the conductance spectrum as a function of the frequency and strength of the field. We found that above a critical value of the field intensity, an enhancement of the conductance, or suppressed resistance, as a function of the field strength occurs. The conductance increases displaying oscillations which amplitude shows a strong dependence on the field frequency. For low radiation energies in comparison with the lead–carbon-nanotube coupling energies, the oscillations evolve toward a structure of well defined steps in the conductance. We have shown that in this range of frequencies, the field intensity dependence of the conductance can give direct information of single-walled carbon nanotube energy spectra.
Carbon nanotubes (CNTs) are nearly ideal one-dimensional (1D) systems, with diameters of only 1–3 nm and lengths that can be on the scale of centimetres. Depending on the arrangement of the carbon-atom honeycomb structure with respect to their axis, CNTs can be direct bandgap semiconductors, or metals with nearly ballistic conduction. The excited states of semiconducting CNTs can be produced by either optical or electrical means and form strongly bound (with dissociation energies of around 0.5 eV), luminescent, 1D excitons. The single-atomic-layer structure makes the optical properties of CNTs especially sensitive to their environment and external fields, and this can be used to tune them. Here we review the nature and properties of CNT excited states, the optical and electrical mechanisms of their production, their radiative and non-radiative modes of decay, the role of external electric fields, and their possible technological use as nanometre-scale light sources, photodetectors and photovoltaic devices.
Carbon nanotubes (CNTs) are amongst the most explored one-dimensional nanostructures and have attracted tremendous interest from fundamental science and technological perspectives. Albeit topologically simple, they exhibit a rich variety of intriguing electronic properties, such as metallic and semiconducting behaviour. Furthermore, these structures are atomically precise, meaning that each carbon atom is still three-fold coordinated without any dangling bonds. CNTs have been used in many laboratories to build prototype nanodevices. These devices include metallic wires, field-effect transistors, electromechanical sensors and displays. They potentially form the basis of future all-carbon electronics.This review deals with the building blocks of understanding the device physics of CNT-based nanodevices. There are many features that make CNTs different from traditional materials, including chirality-dependent electronic properties, the one-dimensional nature of electrostatic screening and the presence of several direct bandgaps. Understanding these novel properties and their impact on devices is crucial in the development and evolution of CNT applications.
We use a transfer-matrix methodology to simulate the rectification of infrared and optical radiation by geometrically asymmetric metal–vacuum–metal junctions in which one of the metals is flat while the other is extended by a tip. We determine in particular the power this junction could provide to an external load and the efficiency with which the energy of incident radiations is converted. We consider first situations in which the external radiation is monochromatic, with typical frequencies in the infrared and optical domains. We then consider situations in which the external radiation consists of a full range of frequencies, with amplitudes that are representative of a focused beam of solar radiation. We investigate in particular how the efficiency of the rectification is affected by the aspect ratio of the tip, the work function of the metallic elements and the occurrence of polarization resonances. Our results demonstrate that the rectification of infrared and optical radiation is possible using devices of the type considered in this work. They finally provide a quantitative analysis of the efficiency of this rectification.
The electronic structure and the conductance of a carbon nanotube based metal/semiconductor/metal intramolecular junction is investigated numerically. The nature of electronic states at the interfaces and in the semiconductor section is analysed. The quantum conductance of the system is calculated in the coherent regime and its variations with energy and length are shown to be related to contributions from different kinds of electronic state.
Single-walled carbon nanotubes provide an ideal system for studying the properties of one-dimensional (1D) materials, where strong electron-electron interactions are expected. Optical measurements have recently reported the existence of excitons in semiconducting nanotubes, revealing the importance of many-body effects. Surprisingly, pioneering electronic structure calculations and scanning tunnelling spectroscopy (STS) experiments report the same gap values as optical experiments. Here, an experimental STS study of the bandgap of single-walled semiconducting nanotubes, demonstrates a continuous transition from the gap reduced by the screening resulting from the metal substrate to the intrinsic gap dominated by many-body interactions. These results provide a deeper knowledge of many-body interactions in these 1D systems and a better understanding of their electronic properties, which is a prerequisite for any application of nanotubes in the ultimate device miniaturization for molecular electronics, or spintronics.
Intramolecular junctions in single-walled carbon nanotubes are potentially ideal structures for building robust, molecular-scale electronics but have only been studied theoretically at the atomic level. Scanning tunneling microscopy was used to determine the atomic structure and electronic properties of such junctions in single-walled nanotube samples. Metal-semiconductor junctions are found to exhibit an electronically sharp interface without localized junction states, whereas a more diffuse interface and low-energy states are found in metal-metal junctions. Tight-binding calculations for models based on observed atomic structures show good agreement with spectroscopy and provide insight into the topological defects forming intramolecular junctions. These studies have important implications for applications of present materials and provide a means for assessing efforts designed to tailor intramolecular junctions for nanoelectronics. Chemistry and Chemical Biology
The excitonic properties of carbon nanotubes embedded in dielectric medium for the range of tube radii and dielectric constants were investigated. The simple scaling relationships showed strong relations between the binding energy, exciton size, and oscillator strength. The approximate calculations show that the effective mass m depends on the tube indices. The exciton size was found to affect directly the observable quantities such as the exciton oscillator strength and the radiative lifetime. The results show the exciton strength is proportional to the probability to find an electron and a hole at the same position.
This letter illustrates the transport phenomena associated with single-walled carbon nanotube (SWNT) junctions of Y- and cross-configurations. Localized gating effect exhibited by Y- and crossed-junctions suggests the resemblance of their electrical characteristics with ambipolar and unipolar p-type FETs, respectively. Temperature dependence of the I-V characteristics reveals that the conduction mechanism in the said SWNT junctions is governed by thermionic emission at temperatures above 100 K and by tunneling at T < 100 K. In-depth analysis of current transport through the crossed- and Y-junction SWNTs is significant in view of their predominant influence on the electrical performance of carbon nanotube networks (CNT-mat).
We develop theoretical arguments that demonstrate the possibility of metallic field-effect transistors (METFET's) in one-dimensional systems and particularly in armchair carbon nanotubes. A very inhomogeneous electric field, such as the field of a tunnelling tip, can penetrate the relatively weakly screened nanotubes and open an energy gap. As a consequence, an energy barrier forms that impedes electron flow and thus permits transistor action. This type of metallic field effect is advantageous because of the high conductance of the metallic tubes in the ON--state. Comment: version from 1/11/04
We calculate the diameter and chirality dependences of the binding energies, sizes, and bright-dark splittings of excitons in semiconducting single-wall carbon nanotubes (SWNTs). Using results and insights from {\it ab initio} calculations, we employ a symmetry-based, variational method based on the effective-mass and envelope-function approximations using tight-binding wavefunctions. Binding energies and spatial extents show a leading dependence with diameter as 1/d and d, respectively, with chirality corrections providing a spread of roughly 20% with a strong family behavior. Bright-dark exciton splittings show a leading dependence. We provide analytical expressions for the binding energies, sizes, and splittings that should be useful to guide future experiments.
Intramolecular junctions in single-walled carbon nanotubes are potentially ideal structures for building robust, molecular-scale
electronics but have only been studied theoretically at the atomic level. Scanning tunneling microscopy was used to determine
the atomic structure and electronic properties of such junctions in single-walled nanotube samples. Metal-semiconductor junctions
are found to exhibit an electronically sharp interface without localized junction states, whereas a more diffuse interface
and low-energy states are found in metal-metal junctions. Tight-binding calculations for models based on observed atomic structures
show good agreement with spectroscopy and provide insight into the topological defects forming intramolecular junctions. These
studies have important implications for applications of present materials and provide a means for assessing efforts designed
to tailor intramolecular junctions for nanoelectronics.
The recent development of the “scanning tunneling microscope” (STM) by Binnig et al. [8.1–5] has made possible the direct real-space imaging of surface topography. In this technique, a metal tip is scanned along the surface while ad justing its height to maintain constant vacuum tunneling current. The result is essentially a contour map of the surface. This contribution reviews the the ory [8.6–8] of STM, with illustrative examples. Because the microscopic structure of the tip is unknown, the tip wave functions are modeled as s-wave functions in the present approach [8.6, 7]. This approximation works best for small effective tip size. The tunneling current is found to be proportional to the surface local density of states (at the Fermi level), evaluated at the position of the tip. The effective resolution is roughly [2Å(R+d)]1/2, where R is the effective tip radius and d is the gap distance. When applied to the 2x1 and 3x1 reconstructions of the Au(l10) surface, the theory gives excellent agreement with experiment [8.4] if a 9 Å tip radius is assumed. For dealing with more complex or aperiodic surfaces, a crude but convenient calculational technique based on atom charge superposition is introduced; it reproduces the Au(l10) results reasonably well. This method is used to test the structure-sensitivity of STM. The Au(l10) image is found to be rather insensitive to the position of atoms beyond the first atomic layer.
Atomically flat, conductive, and transparent noble metal films are produced to extend the wavelength range of room-temperature single-molecule optical absorption detected by scanning tunneling microscopy (SMA-STM). Gold films grown on a platinum underlayer to 15 nm total thickness, deposited by electron beam evaporation onto c-plane sapphire substrates, show sufficient light transmission for backside illumination for laser-assisted STM experiments. Low resistance, transparency, and the atomically flat island surfaces make these good substrates for SMA-STM studies. Monte Carlo lattice kinetics were simulated to allow for a better understanding of the growth modes of the Pt–Au films and of the achieved morphologies. SMA-STM is detected for a quantum dot deposited by aerosol spraying onto Pt–Au films, demonstrating the suitability of such films for single-molecule absorption spectroscopy studies.
We investigate electric-field induced redshifts of photoluminescence from
individual single-walled carbon nanotubes. The shifts scale quadratically with
field, while measurements with different excitation powers and energies show
that effects from heating and relaxation pathways are small. We attribute the
shifts to the Stark effect, and characterize nanotubes with different
chiralities. By taking into account exciton binding energies for air-suspended
tubes, we find that theoretical predictions are in quantitative agreement.
We investigate the effect of temperature tuning on the deposition of gold films onto sapphire. Transparent conductive films with roughness as low as 0.4 angstrom rms at 10 nm thickness allow STM imaging of aerosol-deposited carbon nanotubes. A Monte Carlo lattice model explains the experimentally observed surface-roughening trends and shows that the interplay of deposition-induced kinetic roughening and thermal annealing and roughening can be optimized. Intrinsic surface roughness (e.g., stepping), previously postulated to play a role in film roughness, is not found to be important.
We demonstrate dramatic modifications to the electronic structure in single-walled carbon nanotubes due to band gap renormalization, the many-body induced shrinkage of the fundamental band gap. This is examined within the framework of ideal p-n diodes formed along individual single-walled carbon nanotubes. A combination of photocurrent spectroscopy with detailed transport measurements provides a complete set of energy levels of the nanotube p-n structure. These energy levels confirm the large band gap shrinkage, consistent with enhanced many-body correction in one-dimensional confinement, and result in fundamental changes to the nanotubes diode transport properties as compared to their bulk counterparts. We show that the ideal diode behavior is a direct consequence of significant renormalization of the band gap at the doped p and n regions, resulting in formations of heterointerfaces, in stark contrast to a uniform band gap expected of a homogeneous material.
We have prepared toluene-polymer dispersions of single-walled carbon nanotubes (SWNTs) containing only one or a few specific semiconducting (n,m) species. These have large helical angles and manifest a high discrimination against larger-diameter SWNTs, metallic species, and nanotube aggregates. Such (n,m)-narrowed dispersions proved especially valuable for identification of low-intensity photoluminescence (PL) features as well as relaxation pathways, e.g., for the optically active E22 and E11 excitons, in specific nanotubes. Among other features, PL excitation and emission satellites at ∼(E11+95) and ∼(E11−200) meV, respectively, were resolved. We show that the E22 exciton primarily decays (≥80%) to the optical E11 exciton. A minor decay pathway likely involves relaxation into “dark” deep excitonic states, which weakly emit at ∼40 and ∼140 meV below E11. In addition, we report PL polarization measurements, supporting assignment of transverse E12,21 photoexcitations, and PL quantum yields for (n,m) nanotubes in toluene-polymer dispersions.
The tip-sample capacitance has been studied in the nontunneling regime and the capacitance-distance characteristics and its dependence on the tip geometry have been determined for the gap distance 1<s<600 nm. Measurements were carried out in ultrahigh vacuum on a capacitor formed between a metal tip (W or Pt–Ir) and a clean Au(111) surface. Tips of different tip radius R = 30 ∼ 4000 nm were used to investigate the influence of tip geometry on the capacitance. When the gap distance is reduced, the capacitance increases while its gap sensitivity ∣∂C/∂s∣ decreases with the gap distance. The capacitance therefore shows no 1/s divergence. The magnitude of the capacitance change is found to depend on the tip geometry: blunt tips (R>1000 nm) show larger capacitance increase than that for sharp tips (R ⩽ 100 nm). The effective tip radius Reff estimated from the C−s characteristics agrees with the real tip radius in a limited distance range which varies with the tip geometry. At small distances (s<30 nm), Reff ≃ R for sharp tips but Reff<R for blunt tips. On the other hand at large distances (s>200 nm), the relation is reversed, Reff>R for sharp tips while Reff ∼ R for blunt tips. These results on Reff can be explained by the field concentration to the tip apex and the change of capacitance-contributing tip area with the gap distance. Capacitance calculations indicate that the capacitance of the “truncated cone + half sphere” tip well reproduces the observed C−s characteristics and its dependence on the tip geometry. © 1998 American Institute of Physics.
Single-walled carbon nanotubes (SWNTs) have been studied on a Si(100)-2×1:H surface using an ultrahigh-vacuum (UHV) scanning tunneling microscope (STM). Dry deposition of SWNTs in situ establishes the pristine interface necessary to elucidate fundamental physical and electronic interactions between SWNTs and silicon. We have achieved simultaneous atomic resolution STM images of isolated SWNTs and the local H-passivated Si(100) substrate. Scanning tunneling spectroscopy served to characterize both semiconducting and metallic SWNTs. In each case, electronic features unique to the nanotube can be identified within the substrate band gap. In contrast to previous UHV STM studies of SWNTs on Au(111), our investigation is motivated by the technological relevance of the Si(100) substrate and the potential for nanofabrication of hybrid SWNT-Si electronic devices on the Si(100)-2×1:H platform. © 2003 American Institute of Physics.
We image frequency-modulated single-molecule optical absorption using a scanning tunneling microscope as the detector (SMA−STM). As a first example of the technique, a semiconducting carbon nanotube adsorbed on a silicon surface is studied. Excitation is achieved using laser amplitude as well as frequency modulation, and these two complementary approaches are compared. Detection is achieved via the resulting change in tunneling current through the excited molecule. We distinguish three mechanisms, direct, relaxed, and bolometric, for detecting single-molecule absorption spectra. Kinetic models for these mechanisms as well as for surface heating are presented. The latter effect can be eliminated by frequency modulation, keeping the laser power density on the surface constant.
For the miniaturization of microelectronics, carbon nanotubes (CNTs) are regarded as ideal candidates for the next generation of nanoelectronics because of their excellent properties. To realize CNT‐based electronics, intramolecular junctions are required components, which can not only connect different CNTs for integration, but can also act as functional building blocks in the circuit, such as rectifiers, field‐effect transistors, switches, amplifiers, photoelectrical devices, etc. Therefore, intense attention has been focused on this topic and many advances have been achieved, especially in recent years. On the other hand, some challenges also exist. To provide researchers with a comprehensive overview of this field, this review discusses the synthesis, properties, and applications of intramolecular junctions of CNTs in detail. Among them, the applications of CNT integration are discussed specially. Furthermore, a brief summary and an outlook of future work are provided.
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We calculated the effect of varying the length of a metal-semiconductor carbon nanotube junction on its electrical properties. Joining a metallic (5,5) tube to a semiconducting (10,0) tube leads to the creation of new states near the Fermi energy and produces a larger conductance gap (about 2 eV) than the band gap of the semiconducting segment (about 1 eV). The new states reflect the charge transfer from the (5,5) to the (10,0) segment. The larger conductance gap is due to the mismatch in the conducting states of the (5,5) and (10,0) segments. Although the number of states in the vicinity of E-F increases significantly with increasing nanotube length, the electrical behavior of the junction does not acquire the characteristics of the semiconducting segment. The calculations suggest that the (5,5)/(10,0) nanotube junction could behave as an intrinsic diode for lengths as small as 4 nm.
Nanoscale patterning of the hydrogen terminated Si(100)‐2×1 surface has been achieved with an ultrahigh vacuum scanning tunneling microscope. Patterning occurs when electrons field emitted from the probe locally desorb hydrogen, converting the surface into clean silicon. Linewidths of 1 nm on a 3 nm pitch are achieved by this technique. Local chemistry is also demonstrated by the selective oxidation of the patterned areas. During oxidation, the linewidth is preserved and the surrounding H‐passivated regions remain unaffected, indicating the potential use of this technique in multistep lithography processes.
Four kinds of single-wall carbon nanotubes (SWNTs) with different diameter distribution have been synthesized and optical absorption spectra have been measured. Three large absorption bands due to the optical transitions between spike-like density of states, characteristics of SWNTs, were observed from infrared to visible region. Comparing with the calculated energy band, it has been concluded that the first and the second lowest absorption bands are due to the optical transitions between spikes in semiconductor phases and the third one is due to that in metallic phases. Absorption Peaks sensitively shifted to higher energy side with decreasing tube diameters as the band calculation predicted. Resonance Raman spectra were also measured using various laser lines. When the excitation is in an energy region corresponding to the absorption band of metallic phase, spectra have shown Breit-Wigner-Fano line shape, which is a sign of metallic phase. Using these results, we can easily characterize SWNTs from the optical absorption spectra without Raman measurements and transmission electron microscope observations.
Optical absorption can detect individual molecules and nanostructures even in dissipative or strongly quenching environments where fluorescence signals are weak. Here we image optical absorption of individual carbon nanotubes with subnanometer resolution. We show that we can discriminate adjacent nanotubes on a length scale far below the diffraction limit. Then we compare optical absorption imaging of a defect in a single carbon nanotube (CNT) with conventional scanning tunneling microscopy (STM) and conventional current-voltage scan (I-V) bandgap profiles. We directly visualize the penetration depth σ' = 0.9 ± 0.3 nm of the CNT exciton state into the smaller bandgap region of the defect and derive a size σ = 1.8 ± 0.6 nm for the exciton state. Optical absorption provides a spectroscopic map of molecules simultaneously with conventional STM.
Doping of semiconductors is essential in modern electronic and photonic
devices. While doping is well understood in bulk semiconductors, the advent of
carbon nanotubes and nanowires for nanoelectronic and nanophotonic applications
raises some key questions about the role and impact of doping at low
dimensionality. Here we show that for semiconducting carbon nanotubes, bandgaps
and exciton binding energies can be dramatically reduced upon experimentally
relevant doping, and can be tuned gradually over a broad range of energies in
contrast to higher dimensional systems. The later feature is made possible by a
novel mechanism involving strong dynamical screening effects mediated by
acoustic plasmons.
We present a theory for tunneling between a real surface and a model probe tip, applicable to the recently developed ‘‘scanning tunneling microscope.’’ The tunneling current is found to be proportional to the local density of states of the surface, at the position of the tip. The effective lateral resolution is related to the tip radius R and the vacuum gap distance d approximately as [(2 Å)(R+d)]1/2. The theory is applied to the 2×1 and 3×1 reconstructions of Au(110); results for the respective corrugation amplitudes and for the gap distance are all in excellent agreement with experimental results of Binnig et al. if a 9-Å tip radius is assumed. In addition, a convenient approximate calculational method based on atom superposition is tested; it gives reasonable agreement with the self-consistent calculation and with experiment for Au(110). This method is used to test the structure sensitivity of the microscope. We conclude that for the Au(110) measurements the experimental ‘‘image’’ is relatively insensitive to the positions of atoms beyond the first atomic layer. Finally, tunneling to semiconductor surfaces is considered. Calculations for GaAs(110) illustrate interesting qualitative differences from tunneling to metal surfaces.
Optical transitions in carbon nanotubes are of central importance for nanotube characterization. They also provide insight
into the nature of excited states in these one-dimensional systems. Recent work suggests that light absorption produces strongly
correlated electron-hole states in the form of excitons. However, it has been difficult to rule out a simpler model in which
resonances arise from the van Hove singularities associated with the one-dimensional bond structure of the nanotubes. Here,
two-photon excitation spectroscopy bolsters the exciton picture. We found binding energies of ∼400 millielectron volts for
semiconducting single-walled nanotubes with 0.8-nanometer diameters. The results demonstrate the dominant role of many-body
interactions in the excited-state properties of one-dimensional systems.
We report single molecule laser absorption by carbon nanotubes stamped under ultrahigh vacuum onto Si(100)2x1:H surfaces. Absorption is detected by scanning tunneling microscopy. Images are obtained with and without modulated laser excitation using lock-in amplification and a rear-illumination geometry to reduce thermal effects. Noise appears at topographic edges and is analyzed by a quantitative model in terms of scan speed, mechanical instabilities, and feedback current fluctuations at the edge of the nanotubes. Noise due to mechanical instabilities is shown to persist even in the limit of slow scan speed.
Spectrofluorimetric measurements on single-walled carbon nanotubes (SWNTs) isolated in aqueous surfactant suspensions have revealed distinct electronic absorption and emission transitions for more than 30 different semiconducting nanotube species. By combining these fluorimetric results with resonance Raman data, each optical transition has been mapped to a specific (n,m) nanotube structure. Optical spectroscopy can thereby be used to rapidly determine the detailed composition of bulk SWNT samples, providing distributions in both tube diameter and chiral angle. The measured transition frequencies differ substantially from simple theoretical predictions. These deviations may reflect combinations of trigonal warping and excitonic effects.
Excitonic and free-carrier transitions in single-wall carbon nanotubes are distinguished using field-enhanced photocurrent spectroscopy. Electric field dissociation allows for the detection of bound-exciton states that otherwise would not contribute to the photocurrent. Excitonic states associated with both the ground-state semiconductor and the ground-state metallic nanotube transitions are resolved. The observation of a metallic excitonic state corroborates recent predictions of a symmetry gap existing in metallic nanotubes.
Raman spectroscopy and confocal Raman imaging with 514 nm excitation was performed on recently developed ultralong carbon nanotubes grown by the "fast-heating" chemical vapor deposition (CVD) method. The ultralong nanotubes are found to consist of both semiconducting and metallic types, with spectra that are consistent with the nanotubes being single walled. Characterization of nanotube diameters shows that short nanotubes appearing near the sample catalyst region have a broader distribution than is observed for the ultralong nanotubes. The narrow diameter distribution is determined by uniformity of catalyst particle size and gives additional evidence for the proposed "kite" mechanism for long nanotube growth. Raman imaging was performed over large length scales (up to 140 microm). Imaging reveals the ultralong nanotubes to be of high quality, with a very low defect density. Variations in G-band frequencies and intensity demonstrate the occurrence of minor structural changes and variations in nanotube-substrate interaction along the length of the nanotubes. Evidence also demonstrates that larger structural changes resulting in a full chirality change can occur in these nanotube types to produce a metal-to-semiconductor intramolecular junction.
Single-walled carbon nanotubes (SWNTs) suspended in air over trenches are imaged using their intrinsic near-infrared (NIR) photoluminescence (1.0-1.6 microm). Far-field emission from extended suspended lengths (approximately 50 microm) is both spatially and spectrally resolved, and SWNTs are classified based on the spatial uniformity of their emission intensity and emission wavelength. In a few cases, emission assigned to different (n,m) species is observed along the same suspended segment. Most SWNTs imaged on millisecond time scales show steady emission, but a few fluctuate and suffer a reduction of intensity. The quantum efficiency is dramatically higher than that in previous reports and is estimated at 7%, a value that is precise but subject to corrections because of assumptions about absorption and coherence.
The electronic transition at a metal-semiconductor (M-S) heterojunction within a single-walled carbon nanotubes (SWNT) at the sub-nanometer scale, is characterized through ultrahigh vacuum scanning tunneling microscopy (UHV-STM). STM images show a well-defined physical transition region between semiconducting and metallic nanotube segments while tunneling spectra indicate a gradual transition in SWNT conducting behavior. STM studies of SWNT/III-V systems indicates that charge transfer from the n-InAs(110) surface generally induce p-type behavior within supported semiconducting SWNTs. Scanning tunneling spectroscopy (STS) measurement confirmed the existence of metal-induced gap states (MIGS)originating at the junction interface, with enhanced conductance nanotube segment. Energetically-resolved current image tunneling spectroscopy (CITS) data enabled a quantitative evaluation of the evanscent-state decay.
We calculate the optical properties of carbon nanotubes in an external static
electric field directed along the tube axis. We predict strong Franz-Keldysh
oscillations in the first and second band-to-band absorption peaks, quadratic
Stark effect of the first two excitons, and the field dependence of the bound
exciton ionization rate for a wide range of tube chiralities. We find that the
phonon assisted mechanism dominates the dissociation rate in electro-optical
devices due to the hot optical phonons. We predict a quadratic dependence of
the Sommerfeld factor on the electric field and its increase up to 2000% at the
critical field of the full exciton dissociation.
The field-dependent photocurrent spectrum of individual carbon nanotubes is measured using a displacement photocurrent technique. A series of peaks is observed in the photocurrent corresponding to both excitonic and free carrier transitions. The photocurrent peak corresponding to the ground state exciton increases by a factor of 200 beyond a critical electric field, and shows both red and blue shifts depending on the field regime. This provides evidence for field-induced mixing between excitonic and free carrier states.