Article

Local electrical properties and charging/discharging of CdSe/CdS core-shell nanoplatelets

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Quantum confinement in two-dimensional semiconductor nanoplateletes (NPLs) is determined by their thickness which can be precisely controlled during the synthesis. As a result, NPLs have a very narrow luminescence spectrum and they can provide light sources with very high color purity. Switchable light sources needed for a wide range of applications require the dynamic control of the luminescence. One efficient approach for this purpose is direct charge injection into NPLs. In order to study charging/discharging processes and local electrical properties of CdSe/CdS core-shell NPLs as the model system, here we employed electrical methods based on atomic force microscopy (AFM). Simple and efficient procedures for “write/read/erase” operations are presented: charges are written by a biased AFM tip in contact with the NPLs, their charge state is read by Kelvin probe force or electric force microscopy, whereas injected charges are erased by inversely biased AFM tip. The amount of injected charges is well controlled by a magnitude, polarity and duration of the applied bias voltage, whereas the rate of subsequent spontaneous charge relaxation is dominantly determined by ambient humidity.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... [55] Here, the lessexplored reign of dielectric is nanoscale probing through the facile Kelvin probe force microscopy (KPFM). [55,56] Although, there are a large variety of samples and devices such as conducting polymer, metals, semiconductor devices, Langmuir-Blodgett films, nanoscale photovoltaic mapping, [57] solar cells, [58] memory devices, [59] electrical properties of the functional materials, [60] surface potential of biomolecules, [61] triboelectric nanogenerators, [62] direct determination of density of states, [63] and so on have been analyzed in recent past through KPFM, and established the corresponding contact potential analysis. [64,65] As well, KPFM, also has adequate strength for broad applications in investigating the charge dynamics in ferroelectric and dielectric systems. ...
Article
Full-text available
Metal–organic frameworks (MOFs) have recently attracted a great deal of attention especially as conceivable advanced gate dielectrics for next‐generation field‐effect transistors (FETs) and memory device applications. Dielectric surface charge retention mapping is essential for gauging the leakage current and threshold voltage stability. Due to a dearth of systematic real‐time surface charge probing of MOFs dielectrics, in this work, the nanoscale Kelvin probe force microscopy (KPFM) technique is employed for the contact potential difference (CPD) analysis of developed hybrid copper–metal–organic clusters (Cu‐MOCs)/Si systems. Films are synthesized through the sol‐gel process consisting of copper metal core linked with organic ligands. With 3V positive and negative DC bias, charge injection offset between positive and negative bias is observed to be ≈190 mV, indicating the intrinsic Fermi level alignment between KPFM tip and sample surface charges. The high‐resolution surface CPD mappings bring forth the white (≈98 mV) and black (≈−91 mV) contrast profiles with hole (5.09 × 1012 cm−2) and electron (4.74 × 1012 cm−2) charge densities. Also, the 17 h time‐lapse enables the holes and electrons CPD mapping diameter shrinkage by ≈32% and 46%, respectively. In furtherance, a conductive filament model for host‐guest proton‐assisted conduction with charge hopping through fixed/mobile ions is proposed. Surface charge retention analysis is performed through nanoscale Kelvin probe force microscopy methodology for the dielectric response of functional Cu‐MOCs/Si systems as a reliable gate dielectric application for metal–insulator–semiconductor field‐effect‐transistors complementary metal–oxide–semiconductor (CMOS) technology.
... In the case of nanoplates, their optical properties are closely related to quantum confinement induced by their well-defined thickness equal few monolayers (ML) [3]. Due to this, NPLs have narrow photoluminescence bands (FWHM PL , 7-10 nm), ultrafast PL lifetime (τ PL~2 00-400 ps), and large absorption cross section (10 −14 cm 2 ) [4]. However, due to the shape and crystalline structure defined by surface ligand applied during synthesis, any post-synthesis modification of the NPLs surface should be carried out very carefully [5]. ...
Article
Full-text available
In this paper, the study of surface modification of two-dimensional (2D), non-luminescent CdS nanoplates (NPLs) by thiol-containing ligands is presented. We show that a process of twophase transfers with appropriate ligand exchange transforms non-luminescent NPLs into spherical CdS nanoparticles (NPs) exhibiting a blue photoluminescence with exceptionally high quantum yield ~90%. In the process, transfer from inorganic solvent to water is performed, with appropriately selected ligand molecules and pH values (forward phase transfer), which produces NPs with modified size and shape. Then, in reverse phase transfer, NPs are transferred back to toluene due to surface modification by combined Cd (OL)2 and Cd (Ac)2. As a result, spherical NPs are formed (average diameter between 4 and 6 nm) with PL QY as high as 90%. This is unique for core only CdS NPs without inorganic shell.
... The flexible design and outstanding optical properties of colloidal NPLs have triggered intensive research over the last years aiming at applications in optoelectronic devices. 15,[21][22][23][24] Much of the interest follows from the strong attraction between photogenerated electron-hole pairs, enhanced by the quasi-2D geometry and the dielectric confinement, which prompt large binding energies (150-250 meV) and fast radiative recombination rates through the so-called giant oscillator strength effect. 20,[25][26][27][28] One should however note that the same factors that favor strong electron-hole attraction, favor strong electron-electron or hole-hole repulsion too. ...
Preprint
Colloidal semiconductor nanoplatelets combine weak lateral confinement with strong Coulomb interactions, enhanced by dielectric confinement. When the platelets are charged with carriers of the same sign, this results in severe Coulomb repulsions which shape the electronic structure. To illustrate this point, the shell filling of type-I (CdSe/CdS) and type-II (CdSe/CdTe) core/crown nanoplatelets with up to 4 electrons or holes is investigated theoretically. We find that Coulomb repulsions enable addition energies exceeding room temperature thermal energy and promote the occupation of high-spin states. For charged excitons and biexcitons in CdSe/CdTe nanoplatelets, the repulsions further give rise to multi-peaked emission spectra with widely tunable (over 100 meV) energy, and a transition from type-II to quasi-type-II band profile as the number of electrons confined in the core increases. We conclude that the number of excess carriers injected in nanoplatelets is a versatile degree of freedom to modulate their magnetic and optoelectronic properties.
... The potential difference between the probe tip and sample can be expressed in a difference of work function (W) as follows: eVCPD = Wsample -Wtip, where e is the elementary charge. Therefore, we can obtain the work function of samples by calibrating the probe tip with HOPG, which has a work function of 4.475eV as a reference surface [30][31][32]. Figure 2a,b showed the topography and surface potential image of QDs on SnO2 and ZnO. The uniform surface potential images are due to the evenly spread QD films without pinholes. ...
Article
Full-text available
The performance of colloidal quantum dot light-emitting diodes (QD-LEDs) have been rapidly improved since metal oxide semiconductors were adopted for an electron transport layer (ETL). Among metal oxide semiconductors, zinc oxide (ZnO) has been the most generally employed for the ETL because of its excellent electron transport and injection properties. However, the ZnO ETL often yields charge imbalance in QD-LEDs, which results in undesirable device performance. Here, to address this issue, we introduce double metal oxide ETLs comprising ZnO and tin dioxide (SnO2) bilayer stacks. The employment of SnO2 for the second ETL significantly improves charge balance in the QD-LEDs by preventing spontaneous electron injection from the ZnO ETL and, as a result, we demonstrate 1.6 times higher luminescence efficiency in the QD-LEDs. This result suggests that the proposed double metal oxide ETLs can be a versatile platform for QD-based optoelectronic devices.
Article
Semiconductor CdSe/CdS core-shell nanoplatelets exhibit narrow and intense absorption and photoluminescence spectra in the visible range, which makes them suitable for numerous applications in optoelectronics. Of particular interest is the preparation and optical characterization of thin films with an accurately controlled amount of nanoplatelets. Here we report on the use of spectroscopic ellipsometry for investigating the optical properties of ultrathin films composed of a single layer of negatively charged CdSe/CdS core-shell nanoplatelets prepared by the electrostatic layer-by-layer deposition on SiO 2/Si substrates. Combining the ellipsometric spectra with atomic force microscopy measurements, we were able to infer the nanoplatelet film extinction spectra which was found to exhibit the two characteristic exciton peaks albeit blueshifted relative to the colloidal nanoplatelets.
Article
Full-text available
The lateral dimensions of CdSe nanoplatelets have a strong and unique influence on their opto-electronic properties, with sizes that can be tuned from the weak to the strong exciton confinement regime. There are state-of-the-art reports on several nanoplatelet syntheses; however, at present only the thickness is well-controlled. We demonstrate here that we can achieve a control over the aspect ratio and overall nanoplate area by carefully adjusting the reagents that induce the in-plane growth. A variation of the fraction of hydrated Cd(OAc)2 in a Cd(OAc)2/Cd(OAc)2·2H2O mixture tailors the nanoplatelet aspect ratio. This occurs independently of the reaction time, which can be used to fine-tune the overall length and width. An interpretation is given by the in situ formation of a small amount of hydroxide anions that alter the surface energy of specific planes.
Article
Full-text available
We explore the gold functionalization of 2D CdSe nanoplatelets (NPL) as a possible way to tune their electronic and transport properties. We demonstrate that the size and location of the gold tip can be controlled using light and temperature. The Au tip-CdSe NPL hybrid present a large rise of the conductance compared to the pristine semiconductor (i.e. without gold functionalization). The role of the semiconductor in this transport remains unclear and needs to be better understood. We hypothesize four mechanisms: (i) a reduction of the band gap energy due to the formation of a gold-selenium compound, (ii) a charge transfer between the metal and the semiconductor leading to an increase in carrier concentration and (iii) a change in the inter nanoparticle tunnel barrier height or (iv) a simple percolation process between the metallic grain. X-ray photoelectron spectroscopy (XPS) shows that the CdSe NPL are unaffected by oxidation, and that gold is in the metallic state Au0. We consequently exclude the formation of a narrow band gap Au2Se phase as the possible mechanism leading to the observed rise of conductance. Moreover Kelvin Probe force microscopy and XPS gives evidence for an increase in work function upon gold-tipping, which can be interpreted in terms of a shift of the Fermi level toward the valence band maximum. As hole-conduction in CdSe NPLs is very unlikely to occur, we rather favor the hypothesis that the strong increase in conduction is largely driven by percolation between the metallic tips as the main mechanism responsible for transport in this hybrid system.
Article
Full-text available
We develop the colloidal synthesis and investigate the structural and electronic properties of CdSe-CdTe and inverted CdTe-CdSe heteronanoplatelets and experimentally demonstrate that the overgrowth of cadmium selenide or cadmium telluride core nanoplatelets with counterpartner chalcogenide wings leads to type-II heteronanoplatelets with emission energies defined by the bandgaps of the CdSe and CdTe platelets and the characteristic band offsets. The observed conduction and valence band offsets of 0.36 eV and 0.56 eV are in line with theoretical predictions. The presented type-II heteronanoplatelets exhibit efficient spatially indirect radiative exciton recombination with a quantum yield as high as 23%. While the exciton lifetime is strongly prolonged in the investigated type-II 2D systems with respect to 2D type-I systems, the occurring 2D Giant Oscillator Strength (GOST) effect still leads to a fast and efficient exciton recombination. This makes type-II heteronanoplatelets interesting candidates for low threshold lasing applications and photovoltaics.
Article
Full-text available
Colloidal semiconductor nanocrystals (CS-NCs) possess compelling benefits of low-cost, large-scale solution processing, and tunable optoelectronic properties through controlled synthesis and surface chemistry engineering. These merits make them promising candidates for a variety of applications. This review focuses on the general strategies and recent developments of the controlled synthesis of CS-NCs in terms of crystalline structure, particle size, dominant exposed facet, and their surface passivation. Highlighted are the organic-media based synthesis of metal chalcogenide (including cadmium, lead, and copper chalcogenide) and metal oxide (including titanium oxide and zinc oxide) nanocrystals. Current challenges and thus future opportunities are also pointed out in this review.
Article
Full-text available
By means of scanning probe microscopy we are able to inject charges in isolated graphene sheets deposited on SiO2/Si wafers and characterize the discharge induced by water in controlled ambient conditions. Contact potential differences between the graphene surface and the probe tip, measured by Kelvin probe microscopy, show a linear relationship with the tip bias during charge injection. The discharge depends on relative humidity and decays exponentially with time constants of the order of tens of minutes. We propose that graphene discharges through the water film adsorbed on the SiO2 surface.
Article
Full-text available
Semiconductor nanocrystals have attracted much attention recently due to their unique physical properties and potential use for applications. Despite their importance, the electrostatic properties of semiconductor nanocrystals has received little attention. For example, the presence of electric fields inside a nanocrystal will significantly affect optical, electronic, and electron transport properties. We will present measurements of the dielectric constant and electrostatic charge on single CdSe and CdSe/CdS nanocrystals using electrostatic force microscopy (EFM) in dry air at room temperature. The static dielectric constant of CdSe nanocrystals with diameters ~ 5 nm is uniform. However, the charge per nanocrystal is nonuniform, with some nanocrystals having a positive charge (Q ~ 0.5e). A small fraction of the nanocrystals exhibit a blinking behavior in their charge. This is completely unexpected for a dielectric particle with no additional charge carriers. EFM measurements with simultaneous photoexcitation provide direct evidence of nanocrystal photoionization and increased blinking behavior.
Article
Full-text available
Electrostatic properties of individually separated single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multiwalled carbon nanotubes (MWCNTs) deposited on insulating layers have been investigated by charge injection and electric force microscopy (EFM) experiments. Delocalized charge patterns are observed along the CNTs upon local injection from the EFM tip, corresponding to (i) charge storage in the nanotubes and to (ii) charge trapping in the oxide layer along the nanotubes. The two effects are dissociated easily for CNTs showing abrupt discharge processes in which the charge stored in the CNT are field emitted back to the EFM tip, while trapped oxide charge can subsequently be imaged by EFM, clearly revealing field-enhancement patterns at the CNT caps. The case of continuous discharge processes of SWCNTs, DWCNTs, and MWCNTs is discussed, as well as the evolution of the discharge time constants with respect to the nanotube diameter.
Article
Full-text available
A method to measure the diffusion coefficient of electrons in films of CdSe nanocrystals at room temperature was described. The described method enabled the study of charge transport in films exhibiting extremely high resistances or very small diffusion coefficients. The charge transport was imaged in three-dimensional arrays of CdSe nanocrystal by using a field-effect-transistor geometry. It was shown that the higher spatial and temporal resolution could be achieved by using thinner oxides and small sample areas respectively.
Article
Full-text available
Atomic force microscopy (AFM) and related electrical probe techniques such as electrostatic force microscopy (EFM) can be used to perform injection and detection of electric charge carriers in nanostructures or oxide layer at the nanometer scale. In this paper the control and the deposition of both positive and negative local charges are described and discussed. A basic introduction to both the theoretical and experimental techniques of EFM is also presented. In addition a review of the analytical calculation of tip–surface capacitance interaction is described and then utilised to estimate charge storage in oxide layers and nanostructures from EFM images or spectroscopy curves. The charge resolution of EFM at room temperature and a controlled atmosphere is estimated to be about 20 charge carriers. The EFM technique is also used to inject charges and to study their behaviour in confined silicon nanostructures covered by a thin layer of oxide and separated from the Si substrate by a SiO2 layer. The total injected charge is found to depend on the thickness of the oxide layer. The electric field has emerged to be a key parameter in the injection mechanism. The dynamics and propagation of the deposited charge carriers have been studied and a homogenous distribution of the charge in the nanostructure has been observed. Thanks to these studies and observation, the localisation of the trapped charges has been determined: it occurs mainly in the silicon pattern rather than on the thin covering oxide layer. This charge localisation together with charge energy calculation leads to a better understanding of the origin of the charge dissipation.
Article
Full-text available
The syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still greatly underdeveloped. Here, we demonstrate the formation of atomically flat quasi-two-dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well-defined thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have the electronic properties of two-dimensional quantum wells formed by molecular beam epitaxy, and their thickness-dependent absorption and emission spectra are described very well within an eight-band Pidgeon-Brown model. They present an extremely narrow emission spectrum with full-width at half-maximum less than 40 meV at room temperature. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1 ns at 6 K, two orders of magnitude less than for spherical CdSe nanoparticles. This makes the nanoplatelets the fastest colloidal fluorescent emitters and strongly suggests that they show a giant oscillator strength transition.
Article
Full-text available
Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices.
Article
Full-text available
We report variation of the work function for single and bi-layer graphene devices measured by scanning Kelvin probe microscopy (SKPM). Using the electric field effect, the work function of graphene can be adjusted as the gate voltage tunes the Fermi level across the charge neutrality point. Upon biasing the device, the surface potential map obtained by SKPM provides a reliable way to measure the contact resistance of individual electrodes contacting graphene.
Article
Full-text available
Colloidal semiconductor nanocrystals combine the physical and chemical properties of molecules with the optoelectronic properties of semiconductors. Their colour is highly controllable, a direct consequence of quantum confinement on the electronic states. Such nanocrystals are a form of 'artificial atoms' (ref. 4) that may find applications in optoelectronic systems such as light-emitting diodes and photovoltaic cells, or as components of future nanoelectronic devices. The ability to control the electron occupation (especially in n-type or p-type nanocrystals) is important for tailoring the electrical and optical properties, and should lead to a wider range of practical devices. But conventional doping by introducing impurity atoms has been unsuccessful so far: impurities tend to be expelled from the small crystalline cores (as observed for magnetic impurities), and thermal ionization of the impurities (which provides free carriers) is hindered by strong confinement. Here we report the fabrication of n-type nanocrystals using an electron transfer approach commonly employed in the field of conducting organic polymers. We find that semiconductor nanocrystals prepared as colloids can be made n-type, with electrons in quantum confined states.
Article
It is fundamentally important to understand the nanoscale electronic properties of a single quantum dot (QD) contrary to an ensemble of QDs. Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM) are two important tools, which could be employed to probe surface potential, charging phenomena, and current transport mechanism of individual QD. We demonstrate the aforementioned characteristics of self-assembled Ge QDs, which was grown on Si substrates by solid source molecular beam epitaxy driven by the Stranski-Krastanov method. Study reveals that each Ge QD acts as charge storage node even at zero applied bias. The shape, size and density of QDs could be well probed by CAFM and KPFM, whereas QD facets could be better resolved by the conductive tip. The CAFM investigation further reveals that there exists a composition gradient within the QDs and Ge core can be charged more efficiently than the Si-Ge periphery at low level of bias. Schematic energy band diagram at the tip-sample contact is presented to explain the current transport mechanism, charging/discharging characteristics and the threshold voltages of I–V characteristics of the individual Ge QDs.
Article
The understanding of local charge trapping on the nanoscale is crucial for the design of novel electronic devices and photodetectors based on SiGe nanoclusters (NCs). Here, the local spatial distribution of the surface potential of the Ge NCs was detected using Kelvin probe force microscopy (KPFM). Different surface potentials between Ge NCs and the wetting layer (WL) surface were detected at room temperature. Changes of the local contact potential differences (CPD) were studied after injection of electrons or holes into single Ge NCs on top of the Si layer using a conductive atomic force microscopy tip. The CPD image contrast was increased after electron injection by applying a forward bias to the n-tip/i-Ge NC/p-Si junction. Injecting holes into a single Ge NC was also accompanied by filling of two-dimensional states in the surrounding region, which is governed by leakage currents through WL or surface states and Coulomb charging effects. A long retention time of holes trapped by the Ge NC was found.
Chapter
In this chapter we overview recent achievements in the synthesis and investigation of optical properties of the new type of quantum-sized semiconductor nanocrystals – 2D nanoplatelets. Thin, atomically flat AIIBVI nanoplatelets prepared by colloidal chemistry routes are good model objects for studying optical properties of 2D semiconductor structures. Due to their discrete thickness, nanoplatelets exhibit spectrally narrow excitonic optical transitions in absorption and photoluminescence. Large lateral size of semiconductor nanoplatelets results in very efficient light absorption with the molar absorption coefficient an order of magnitude greater than that of 0D quantum dots and 1D nanorods. Sharp excitonic absorption bands of CdSe nanoplatelets allow observation of large electro-optical Quantum Confined Stark effect which is much stronger than in the corresponding nanorods, or quantum dots. Besides binary core CdSe, CdS, CdTe nanoplatelets, more complex CdSe/CdS core-shell and unique core-wings CdSe-CdS, CdSe-CdTe and CdTe-CdSe heteronanoplatelets with type I and type II optical transitions and bright photoluminescence can be synthesized. This expands the field of potential practical applications of nanoplatelets as efficient light convertors and fluorescent markers even further.
Article
Over the last several years tremendous progress has been made in incorporating Colloidal Quantum Dot (CQD) solids as photoactive components in optoelectronic devices. A large part of that progress is associated with significant advancements made in controlling the electronic doping of CQD solids. Today a variety of strategies exists towards that purpose; this minireview aims to survey major published works in this subject. Additional attention is given to the many challenges associated with the task of doping CQDs, as well as to the optoelectronic functionalities and applications being realized when successfully achieving light and heavy electronic doping of CQD solids.
Article
We report on the fabrication of a hybrid light-emitting-diode based on colloidal semiconductor CdSe nanoplatelets as emitters and organic TAZ [3-(Biphenyl-4-yl)-5-(4-tea-butylpheny1)-4-pheny1-4H-1,2,4triazole] and TPD [N, N-bis (3-methylpheny1)-N, N-bis (phenyl)-benzidine] materials as the electron and hole transporting layers. Electroluminescent and current-voltage characteristics of the developed hybrid device with the turn-on voltage of 5.5V and the radiation wavelength of 515 nm have been obtained. Semiconductor nanoplatelets like CdSe are attractive for the fabrication of hybrid LEDs with low operating voltages, spectrally pure color and short-wavelength electroluminescence, which is required for RGB devices.
Article
Colloidal nanoplatelets, quasi-two-dimensional quantum wells, have recently been introduced as colloidal semiconductor materials with the narrowest known photoluminescence linewidth (~10 nm). Unfortunately, these materials have not been shown to have continuously-tunable emission, but rather emit at discrete wavelengths that depend strictly on atomic-layer thickness. Herein, we report a new synthesis approach that overcomes this issue: by alloying CdSe colloidal nanoplatelets with CdS, we finely-tune the emission spectrum while still leveraging atomic-scale thickness control. We proceed to demonstrate light emitting diodes with sub-bandgap turn-on voltages (2.1 V for a device emitting at 2.4 eV) and the narrowest electroluminescence spectrum (FWHM ~12.5 nm) reported for colloidal semiconductor LEDs.
Article
Graphene oxide (GO) has emerged as a multifunctional material that can be synthesized in bulk quantities and can be solution processed to form large-area atomic layered photoactive, flexible thin films for optoelectronic devices. This is largely due to the potential ability to tune electrical and optical properties of GO using functional groups. For the successful application of GO, it is key to understand the evolution of its optoelectronic properties as the GO undergoes a phase transition from its insulating and optically active state to the electrically conducting state with progressive reduction. In this paper, we use a combination of electrostatic force microscopy (EFM) and optical spectroscopy to monitor the emergence of the optoelectronic properties of GO with progressive reduction. EFM measurements enable, for the first time, direct visualization of charge propagation along the conducting pathways that emerge on progressively reduced graphene oxide (rGO) and demonstrate that with the increasing degree of reduction, injected charges can rapidly migrate over a distance of several micrometers, irrespective of their polarities. Direct imaging reveals the presence of an insurmountable potential barrier between reduced GO (rGO) and GO, which plays the decisive role in the charge transport. We complement charge imaging with theoretical modeling using quantum chemistry calculations that further demonstrate that the role of barrier in regulating the charge transport. Furthermore, by correlating the EFM measurements with photoluminescence imaging and electrical conductivity studies, we identify a bifunctional state in GO, where the optical properties are preserved along with good electrical conductivity, providing design principles for the development of GO-based, low-cost, thin-film optoelectronic applications.
Article
The nanoscale imaging of charge flow in proteins is crucial to understanding several life processes, including respiration, metabolism and photosynthesis. However, existing imaging methods are only effective under non-physiological conditions or are limited to photosynthetic proteins. Here, we show that electrostatic force microscopy can be used to directly visualize charge propagation along pili of Geobacter sulfurreducens with nanometre resolution and under ambient conditions. Charges injected at a single point into individual, untreated pili, which are still attached to cells, propagated over the entire filament. The mobile charge density in the pili, as well as the temperature and pH dependence of the charge density, were similar to those of carbon nanotubes and other organic conductors. These findings, coupled with a lack of charge propagation in mutated pili that were missing key aromatic amino acids, suggest that the pili of G. sulfurreducens function as molecular wires with transport via delocalized charges, rather than the hopping mechanism that is typical of biological electron transport.
Article
Atomic layer deposition (ALD) is widely used for gas-phase deposition of high-quality dielectric, semiconducting, or metallic films on various substrates. In this contribution we propose the concept of colloidal ALD (c-ALD) for synthesis of colloidal nanostructures. During the c-ALD process, either nanoparticles or molecular precursors are sequentially transferred between polar and nonpolar phases to prevent accumulation of unreacted precursors and byproducts in the reaction mixture. We show that binding of inorganic ligands (e.g., S2–) to the nanocrystal surface can be used as a half-reaction in c-ALD process. The utility of this approach has been demonstrated by growing CdS layers on colloidal CdSe nanocrystals, nanoplatelets, and CdS nanorods. The CdS/CdSe/CdS nanoplatelets represent a new example of colloidal nanoheterostructures with mixed confinement regimes for electrons and holes. In these materials holes are confined to a thin (1.8 nm) two-dimensional CdSe quantum well, while the electron confinement can be gradually relaxed in all three dimensions by growing epitaxial CdS layers on both sides of the quantum well. The relaxation of the electron confinement energy caused a shift of the emission band from 510 to 665 nm with unusually small inhomogeneous broadening of the emission spectra.
Article
Colloidal nanoplatelets (NPLs) have recently emerged as favorable light-emitting materials, which also show great potential as optical gain media owing to their remarkable optical properties. In this work, we systematically investigate the optical gain performance of CdSe-core and CdSe/CdS-core/crown NPLs having different CdS-crown size with one- and two-photon absorption pumping. The core/crown NPLs exhibit enhanced gain performance as compared to the core only NPLs owing to increased absorption cross-section and the efficient inter-exciton funneling, which is from the CdS-crown to the CdSe-core. One- and two-photon absorption pumped amplified spontaneous emission thresholds are found as low as 41 µJ/cm2 and 4.48 mJ/cm2, respectively. These thresholds surpass the best reported optical gain performance of the state-of-the-art colloidal nanocrystals (i.e., quantum dots, nanorods, etc.) emitting in the same spectral range as the NPLs. Moreover, gain coefficient of the NPLs is measured as high as 650 cm-1, which is 4-fold larger than the best reported gain coefficient of the colloidal quantum dots. Finally, we demonstrate a two-photon absorption pumped vertical cavity surface emitting laser of the NPLs with a lasing threshold as low as 2.49 mJ/cm2. These excellent results are attributed to the superior properties of the NPLs as optical gain media.
Article
The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets (NPLs) produce amplified spontaneous emission with thresholds as low as 6 µJ/cm2 and gain as high as 600 cm-1, both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences two orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission linewidths.
Article
The first functional light‐emitting diodes (LEDs) based on quasi 2D colloidal core/shell CdSe/CdZnS nanoplatelets (NPLs). The solution‐processed hybrid devices are optimized with respect to their electroluminescent characteristics, first, by improving charge injection through exchanging the as‐synthesized NPL long‐chain ligands to shorter ones such as 3‐mercaptopropionic acid, and second, by comparing different hole‐transporting layers. NPL‐LEDs exhibit a maximum luminance of 4499 cd m‐2 and external quantum efficiencies of 0.63%. In particular, over different applied voltages, systematically narrow electroluminescence of full width at half maximum (FWHM) in the range of 25–30 nm is observed to be independent from the choice of device configuration and NPL ligands. As spectrally narrow electroluminescence is highly attractive in terms of color purity in the context of LED applications, these results emphasize the unique potential of this new class of colloidal core/shell nanoplatelet in achieving bright and functional LEDs of superior color purity. The first light‐emitting diodes (LEDs) based on quasi‐2D colloidal core/shell CdSe/CdZnS nanoplatelets (NPLs) under a solution‐processed hybrid device structure are reported. Over different applied voltages, systematically narrow electroluminescence of FWHM in the range of 25–30 nm is observed, which is highly attractive in terms of color purity in the context of LED applications.
Article
The charge state of free standing axial type-II CdSe/CdTe hetero-nanowires is monitored via electrostatic force microscopy. The CdSe and the CdTe segment which are identified by Raman spectroscopy are found to be negatively and positively charged, respectively. The charge state is monitored without and with local illumination. We found that the magnitude of opposite charging in the respective nanowire segment is increasing with illumination power, which is attributed to a charge separation of the photogenerated electron-hole pairs across the CdSe/CdTe interface.
Article
Since their inception 18 years ago, electrically driven colloidal quantum-dot light-emitting devices (QD-LEDs) have increased in external quantum efficiency from less than 0.01% to around 18%. The high luminescence efficiency and uniquely size-tunable colour of solution-processable semiconducting colloidal QDs highlight the potential of QD-LEDs for use in energy-efficient, high-colour-quality thin-film display and solid-state lighting applications. Indeed, last year saw the first demonstrations of electrically driven full-colour QD-LED displays, which foreshadow QD technologies that will transcend the optically excited QD-enhanced lighting products already available today. We here discuss the key advantages of using QDs as luminophores in LEDs and outline the operating mechanisms of four types of QD-LED. State-of-the-art visible-wavelength LEDs and the promise of near-infrared and heavy-metal-free devices are also highlighted. As QD-LED efficiencies approach those of molecular organic LEDs, we identify the key scientific and technological challenges facing QD-LED commercialization and offer our outlook for on-going strategies to overcome these challenges.
Article
Electronic properties of individual CdSe colloidal nanodots have been investigated by conductive-tip atomic force microscopy (AFM). Submonolayer-thick films of the colloidal nanodots were fabricated on a self-assembled monolayer of alkanethiol molecules formed on Au(111) surfaces for single dot measurements. First, we simultaneously imaged the topography and conductivity of isolated single dots by AFM operating in contact mode with a conductive tip under appropriate bias voltages. In the current image, it is found that the dot regions have higher electric resistances due to tunneling resistance through the CdSe dots. We found a 10-nm scale electric inhomogeneity around the dots, which may correspond to the previously reported etch-pits of Au(111) surfaces formed during the deposition of the alkanethiol molecules. Then, current–voltage characteristics were measured with the conductive tip positioned on the single dots; large changes in the conductivity which suggest resonant tunneling through the quantized energy level in the dot were observed even at room temperature.
Article
ELECTROLUMINESCENT devices have been developed recently that are based on new materials such as porous silicon1 and semiconducting polymers2,3. By taking advantage of developments in the preparation and characterization of direct-gap semiconductor nanocrystals4-6, and of electroluminescent polymers7, we have now constructed a hybrid organic/inorganic electroluminescent device. Light emission arises from the recombination of holes injected into a layer of semiconducting p-paraphenylene vinylene (PPV)8-10 with electrons injected into a multilayer film of cadmium selenide nanocrystals. Close matching of the emitting layer of nanocrystals with the work function of the metal contact leads to an operating voltage11 of only 4V. At low voltages emission from the CdSe layer occurs. Because of the quantum size effect19-24 the colour of this emission can be varied from red to yellow by changing the nanocrystal size. At higher voltages green emission from the polymer layer predominates. Thus this device has a degree of voltage tunability of colour.
Article
Applying conductance atomic force microscopy to measure the local current routes in ensembles of CdSe nanocrystallites of diameters in the range of 8–12 nm, we found that the electrical transport takes place through the crystallites themselves rather than along their grain boundaries. Statistical analysis of the current images, in correlation with macroscopic electrical transport measurements, reveals a crossover between two different conduction regimes. The behavior in each regime is found to be governed by the average diameter of the nanocrystals in the ensemble. We interpret the above findings as a transport manifestation of the quantum confinement effect.
Article
Incorporating nanocrystals into future electronic or optoelectronic devices will require a means of controlling charge-injection processes and an understanding of how the injected charges affect the properties of nanocrystals. We show that the optical properties of colloidal semiconductor nanocrystal quantum dots can be tuned by an electrochemical potential. The injection of electrons into the quantum-confined states of the nanocrystal leads to an electrochromic response, including a strong, size-tunable, midinfrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions, and a quench of the narrow band-edge photoluminescence.
Article
We have studied charge injection and charge transport in thin disordered films of CdSe nanocrystals between metal electrodes. Current–voltage characteristics of these devices are investigated as a function of electrode material, nanocrystal size, and temperature. We measure the photocurrent response of these structures and find that the photocurrent action spectra follow the quantum-confined absorption spectra of the nanoparticles. For dissimilar top and bottom electrodes, we find that the devices are highly rectifying. High work function materials such as gold and indium-tin oxide are found to be poor electron injectors, consistent with the estimated conduction and valence band levels of the nanocrystals. We observe that the current–voltage characteristics exhibit a history and time dependence which is characteristic of persistent photoconductivity, with current at constant bias decaying with time according to a stretched exponential form. We propose a model based on space-charge limited current dominated by mobile electrons which slowly fill deep traps. Numerical simulations show that the model is able to describe the observed time dependence. We also find that the conductivity is strongly temperature dependent, and is qualitatively consistent with an activated hopping process at temperatures above 180 K. We use the data and simulations to estimate the electron mobilities to be in the range of ∼10−4–10−6 cm2 V−1 s−1 and the trap densities to be approximately 2×1016 cm−3.© 2000 American Institute of Physics.
Article
Tapping mode atomic force microscopy (AFM) is employed for dynamic plowing lithography of exfoliated graphene on silicon dioxide substrates. The shape of the graphene sheet is determined by the movement of the vibrating AFM probe. There are two possibilities for lithography depending on the applied force. At moderate forces, the AFM tip only deforms the graphene and generates local strain of the order of 0.1%. For sufficiently large forces the AFM tip can hook graphene and then pull it, thus cutting the graphene along the direction of the tip motion. Electrical characterization by AFM based electric force microscopy, Kelvin probe force microscopy and conductive AFM allows us to distinguish between the truly separated islands and those still connected to the surrounding graphene.
Article
This article summarizes the main achievements and challenges in the field of the aqueous synthesis of semiconductor quantum dots in colloidal solutions. Developments in the last two decades demonstrate the great potential of this approach to synthesize nanocrystalline materials with superior properties such as strong photoluminescence, long time stability and compatibility with biological media, and the variability in assembling and self-assembling into larger structures or on surfaces. Being relatively straightforward, the aqueous approach provides some advantages such as versatility, scalability, environmental friendliness and cost effectiveness, leading in summary to very attractive application perspectives.
Article
We have recently synthesized atomically flat semiconductor colloidal nanoplatelets with quasi 2D geometry. Here, we show that core/shell nanoplatelets can be obtained with a 2D geometry that is conserved. The epitaxial growth of the shell semiconductor is performed at room temperature. We report the detailed synthesis of CdSe/CdS and CdSe/CdZnS structures with different shell thicknesses. The shell growth is characterized both spectroscopically and structurally. In particular, the core/shell structure appears very clearly on high-resolution high-angle annular dark-field transmission electron microscope images, thanks to the difference of atomic density between the core and the shell. When the nanoplatelets stand on their edge, we can precisely count the number of atomic planes forming the core and the shell. This provides a direct measurement, with atomic precision, of the core nanoplatelets thickness. The constraints exerced by the shell growth on the core is analyzed using global phase analysis. The core/shell nanoplatelets we obtained have narrow emission spectra with full-width half-maximum close to 20 nm, and quantum yield that can reach 60 %.
Article
The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future research efforts toward utilization of tailored nanostructures will be an important challenge for the development of next generation solar cells.
Article
A quantitative study of the absorption and photoluminescence (PL) of charged CdSe/CdS nanocrystal films is reported. Unlike CdSe, where the PL was unstable under reducing condition, the CdSe/CdS nanocrystals can be kept under reducing bias in their charged state for tens of minutes, and the PL intensity can be switched quickly and reversibly. Thin films of CdSe/CdS quantum dots are stably reduced to −IV oxidation state without evidence for surface charges or generation of surface nonradiative recombination sites. Upon charging, the quantum dot films exhibit rapid and reversible switching of photoluminescence intensity as well as stable photoluminescence characteristics. The extent of PL quench is not just dependent on the number of carriers but also on their quantum states. The 1Pe electron quenches the luminescence 8 times more than the 1Se electron. The CdSe/CdS nanocrystals also exhibit reduced threshold for stimulated emission in their charged state.
Article
The electrostatic charge and photoionization characteristics of 5-nm CdSe nanocrystals were directly observed with electrostatic force microscopy (EFM) in dry air at room temperature. Measurements were made on individual nanocrystals, as well as on those in self-assembled rafts. Nanocrystals are initially charge neutral if protected from sources of light. However, over a few weeks some nanocrystals develop a single positive charge if exposed to ambient light. The determination of the charge magnitude per nanocrystal within the framework of EFM theory is described. EFM measurements with simultaneous above band gap laser photoexcitation provide direct evidence of nanocrystal photoionization. A small percentage of photoionized nanocrystals exhibit a blinking behavior in their charge. The linear dependence of nanocrystal photoionization rates on excitation intensity indicates that the ionization process occurs via a single photon. EFM measurements of core/shell CdSe nanocrystals show that photoionization is slower in the presence of an electron barrier at the nanocrystal surface. Photoionization and subsequent neutralization are quantitatively modeled with a two-level system.
Article
CdSe nanowires show reversible emission intensity enhancements when subjected to electric field strengths ranging from 5 to 22 MV/m. Under alternating positive and negative biases, emission intensity modulation depths of 14 ± 7% are observed. Individual wires are studied by placing them in parallel plate capacitor-like structures and monitoring their emission intensities via single nanostructure microscopy. Observed emission sensitivities are rationalized by the field-induced modulation of carrier detrapping rates from NW defect sites responsible for nonradiative relaxation processes. The exclusion of these states from subsequent photophysics leads to observed photoluminescence quantum yield enhancements. We quantitatively explain the phenomenon by developing a kinetic model to account for field-induced variations of carrier detrapping rates. The observed phenomenon allows direct visualization of trap state behavior in individual CdSe nanowires and represents a first step toward developing new optical techniques that can probe defects in low-dimensional materials.
Article
We study the electronic structure of ultrathin zinc-blende two-dimensional (2D)-CdSe nanosheets both theoretically, by Hartree-renormalized k·p calculations including Coulomb interaction, and experimentally, by temperature-dependent and time-resolved photoluminescence measurements. The observed 2D-heavy hole exciton states show a strong influence of vertical confinement and dielectric screening. A very weak coupling to phonons results in a low phonon-contribution to the homogeneous line-broadening. The 2D-nanosheets exhibit much narrower ensemble absorption and emission linewidths as compared to the best colloidal CdSe nanocrystallites ensembles. Since those nanoplatelets can be easily stacked and tend to roll up as they are large, we see a way to form new types of multiple quantum wells and II-VI nanotubes, for example, for fluorescence markers.
Article
Semiconductor nanocrystals exhibit a wide range of size-dependent properties. Variations in fundamental characteristics ranging from phase transitions to electrical conductivity can be induced by controlling the size of the crystals. The present status and new opportunities for research in this area of materials physical chemistry are reviewed. 78 refs., 21 figs.
Article
An investigation of the effect of an applied electric field on the photoluminescence (PL) intensity of single CdSe nanocrystals has revealed a measurable field induced PL modulation for a large fraction of the nanocrystals studied. The field induced intensity modulation characteristics (i.e. modulation sign and depth) were observed to vary from particle to particle, and even for different time periods for the same particle in many cases. Simultaneous intensity and frequency resolved PL measurement show that the PL intensity modulation is in fact due to an electric field effect on the PL quantum yield. The results are consistent with a model in which the energies of surface charge trapping sites are modulated by the applied electric field, causing in turn a modulation of the rates of exciton quenching by these sites. The complex observed field effects can be explained by the superposition of the applied and internal electric fields due to deeply trapped charges on the surface of the nanoparticle.
Article
The low-intensity photoionization of individual semiconductor nanocrystals, at 23 Deg in dry nitrogen, is time-resolved over many hours for both S (532-nm excitation) and P (395-nm excitation) nanocrystal excited states using electrostatic force microscopy. Over 7000 calibrated charge measurements have been made on 14- and 21-.ANG.-thick oxide layers. Photoexcited electrons tunnel across the oxide into the silicon, and multiple charges can build up on individual nanocrystals at intensities of only 0.1-0.01 W/cm2. The silicon dopant type influences the net nanocrystal charging via the interfacial band bending; P-type substrates show a faster nanocrystal reneutralization rate due to their higher interfacial electron concn. There is a huge range of photoionization behavior for individual nanocrystals. This behavior is different for 395- and 532-nm excitation in the same nanocrystal. This individuality seems in part to reflect tunneling through spatially localized defect states in the oxide. The line widths of spatial charge images of individual nanocrystals and the semicontinuous rate of charge re-neutralization after excitation suggest that we observe trapped electron motion in the adjacent oxide and/or on the nanocrystal surface, in addn. to the ionized nanocrystal. On av., tunneling of the excited P electron is faster by 1-2 orders of magnitude than that of the S electron; the data show direct photoionization from the excited P state. A kinetic model is developed, including the effect of charging energy on tunneling rate, and applied to ensemble av. behavior. There is no quant. agreement of the tunneling-rate dependence on oxide thickness and excitation energy with the simple 1D effective mass tunneling model. However, overall obsd. trends are rationalized in light of current thin-oxide tunneling literature. [on SciFinder (R)]
Article
We demonstrate a direct correlation between the charge state and photoluminescence (PL) intensity of individual CdSe nanowires by actively charging them and performing electrostatic force microscopy and PL measurements simultaneously. While the injection of positive charges leads to an immediate PL quenching, a small amount of injected electrons can lead to an increase of the PL intensity. We directly observed the migration of excess charges into the substrate, which leads to a recovery of the PL. Further, we show that the PL of individual NWs can be actively switched between on and off states by charging with the atomic-force microscope tip. We propose a model based on charge trapping and migration into the substrate to explain our results.
Article
A combination of electrostatic force microscopy and optical microscopy was used to investigate the charge state of individual CdSe nanowires upon local illumination with a focused laser beam. The nanowires were found to be positively charged at the excitation spot and negatively charged at the distant end(s). For high laser powers, the amount of accumulated charges increases logarithmically with the laser power. These effects are described by a diffusion-based model where the results are in good agreement with the experimentally observed effects. On the basis of this model the charge imbalance along the nanowire should establish in the course of nanoseconds. The net charge separation within homogeneous nanowires upon local illumination is of importance for several electronic devices.
Article
Using micro-Raman spectroscopy and scanning tunneling microscopy, we study the relationship between structural distortion and electrical hole doping of graphene on a silicon dioxide substrate. The observed upshift of the Raman G band represents charge doping and not compressive strain. Two independent factors control the doping: (1) the degree of graphene coupling to the substrate and (2) exposure to oxygen and moisture. Thermal annealing induces a pronounced structural distortion due to close coupling to SiO(2) and activates the ability of diatomic oxygen to accept charge from graphene. Gas flow experiments show that dry oxygen reversibly dopes graphene; doping becomes stronger and more irreversible in the presence of moisture and over long periods of time. We propose that oxygen molecular anions are stabilized by water solvation and electrostatic binding to the silicon dioxide surface.
Article
Semiconductor nanocrystals are tiny light-emitting particles on the nanometer scale. Researchers have studied these particles intensely and have developed them for broad applications in solar energy conversion, optoelectronic devices, molecular and cellular imaging, and ultrasensitive detection. A major feature of semiconductor nanocrystals is the quantum confinement effect, which leads to spatial enclosure of the electronic charge carriers within the nanocrystal. Because of this effect, researchers can use the size and shape of these "artificial atoms" to widely and precisely tune the energy of discrete electronic energy states and optical transitions. As a result, researchers can tune the light emission from these particles throughout the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges. These particles also span the transition between small molecules and bulk crystals, instilling novel optical properties such as carrier multiplication, single-particle blinking, and spectral diffusion. In addition, semiconductor nanocrystals provide a versatile building block for developing complex nanostructures such as superlattices and multimodal agents for molecular imaging and targeted therapy. In this Account, we discuss recent advances in the understanding of the atomic structure and optical properties of semiconductor nanocrystals. We also discuss new strategies for band gap and electronic wave function engineering to control the location of charge carriers. New methodologies such as alloying, doping, strain-tuning, and band-edge warping will likely play key roles in the further development of these particles for optoelectronic and biomedical applications.
Article
Electrostatic force microscopy is used to study light-induced charging in single hybrid Au-CdSe nanodumbbells. Upon illumination, nanodumbbells show negative charging, which is in contrast with CdSe rods and Au particles that show positive charging. This different behavior is attributed to charge separation in the nanodumbbells, where after excitation the electron is transferred to the gold tips and the hole is subsequently filled through tunneling interactions with the substrate. The process of light-induced charge separation at the metal-semiconductor interface is key for the photocatalytic activity of such hybrid metal-semiconductor nanostructures.
Article
We use electric force microscopy (EFM) to study single nanocrystal photoionization in two classes of high-quality nanocrystals whose exciton luminescence quantum yields approach unity in solution. The CdSe/CdS/ZnS core/shell nanocrystals do not photoionize, while the CdSe/CdS nanocrystals do show substantial photoionization. This verifies the theoretical prediction that the ZnS shell confines the excited electron within the nanocrystal. Despite the high luminescence quantum yield, photoionization varies substantially among the CdSe/CdS nanocrystals. We have studied the nanocrystal photoionization with both UV (396 nm) and green (532 nm) light, and we have found that the magnitude of the charge due to photoionization per absorbed photon is greater for UV excitation than for green excitation. A fraction of the photoionization occurs directly via a "hot electron" process, using trap states that are either on the particle surface, within the ligand sphere, or within the silicon oxide layer. This must occur without relaxation to the thermalized, lowest-energy, emitting exciton. We discuss the occurrence of hot carrier processes that are common to photoionization, luminescence blinking, and the fast transient optical absorption that is associated with multiple exciton generation MEG studies.
Article
The conductance of isolated CdSe quantum dots (QD's) of ~4 nm diameter, epitaxially electrodeposited on gold substrates, has been studied using a scanning force microscope with a metallized tip to perform current-voltage spectroscopy. The band gaps of the dots have been measured and correlated with the QD size distribution. Reproducible peaks in the room-temperature conductance spectra are interpreted as transport of single electrons through the quantum dots. The roles of both Coulomb charging and interlevel spacing are considered.