Charge Puddles Research Articles

Existence of electron–hole charge puddles and the observation of strong universal conductance fluctuations in a 3D topological insulator

Phase-coherent transport manifests itself in various quantum phenomena. Universal conductance fluctuations (UCF) are one of them. Here, we report the detailed study of dephasing of electronic wavefunction by analyzing UCF and weak antilocalization in a device, made by exfoliation from a bulk insulating topological insulator single crystal (Bi1Sb1Te1.5Se1.5). From the parallel magnetic field measurement, we proved that the quantum phenomena in our sample are caused by surface Dirac electrons. Our sample showed very slow dephasing of electronic wavefunction, which has rarely been observed in topological insulator materials. This kind of dephasing indicates the presence of electron–hole charge puddles in our system. The RMS amplitude of the UCF is also surprisingly higher (13 e2h) than the universal value of the conductance fluctuation (e2h) and the fluctuations persist in our sample until the temperature as high as 20 K and above. We also show that the shape of the flake has a significant impact on the prefactor, α, of the Hikami–Larkin–Nagaoka equation. Moreover, the formation of Aharonov–Bohm loops of different areas is responsible for UCF. By analyzing the fast Fourier transform of the UCF data, we have obtained these Aharonov–Bohm loops which are contributing to the UCF.

Double quantum dot-like transport in controllably doped graphene nanoribbon

In this Letter, we demonstrate coupled double-quantum dot (DQD)-like transport in an ∼30 nm-wide controllably doped graphene nanoribbon (GNR). Controlled doping is introduced from hydrogen silsesquioxane by changing its electron exposure dose. The proximity effect, which brings in additional dose accumulation, is utilized to introduce two charge puddles with stronger p-doping at the two ends of the moderately p-doped GNR, which act as two quantum dots. By electrostatically isolating these two charge puddles with simplified overlapping dual gates, DQD-like transport features are measured in the doped GNR at a temperature of 5 K. Moreover, the transition from strongly to weakly coupled DQDs is observed due to electrically tunable inter-dot coupling.

Evolution of Charge and Pair Density Modulations in Overdoped Bi2Sr2CuO6+δ

One of the central issues concerning the mechanism of high temperature superconductivity in cuprates is the nature of the ubiquitous charge order and its implications to superconductivity. Here we use scanning tunneling microscopy to investigate the evolution of charge order from the optimally doped to strongly overdoped Bi2Sr2CuO6+{\delta} cuprates. We find that with increasing hole concentration, the long-range checkerboard order gradually evolves into short-range glassy patterns consisting of diluted charge puddles. Each charge puddle has a unidirectional nematic internal structure, and exhibits clear pair density modulations as revealed by the spatial variations of superconducting coherence peak and gap depth. Both the charge puddles and the nematicity vanish completely in the strongly overdoped non-superconducting regime, when another type of short-range order with root2 * root2 periodicity emerges. These results shed important new lights on the intricate interplay between the intertwined orders and the superconducting phase of cuprates.

Observation of dopant-dependent efficiency in chemically doped graphene/silicon solar cells and prospects for MoOx to overcome the stability and efficiency limits

Heterointegration of graphene with silicon has attracted growing interest, because the resulting unique heterojunction allows for efficient collection of light generated electron–hole pairs. This study aims to understand the role of device fabrication conditions and electronic properties of chemically doped graphene on the efficiency variations and stability of graphene/silicon solar cells. We observed significant variations in the efficiency between the devices doped with metal chloride and organic molecules. A strong degradation in the hole carrier mobility by the formation of metal particles/clusters, which could act like charge puddles, accounts for the limited efficiency in the former. Molecular doping, on the other hand, offered good doping homogeneity and no mobility degradation, leading to solar cells with efficiency as high as 9.2%. Our results demonstrate that the droop in efficiency over time observed in the chemically doped devices is due to oxidation limited charge carrier separation rather than doping reversal. The prospects of molybdenum oxide (MoOx) as a multifunctional layer in improving the device stability and efficiency are also discussed based on preliminary experimental findings.

Landscape of Charge Puddles in Graphene Nanoribbons on Hexagonal Boron Nitride

Recently, graphene nanoribbons (GNRs) on hexagonal boron nitride (h‐BN) substrates have been studied to develop high‐mobility devices or devices based on a 1D Moiré superlattice. For this purpose, a device‐level understanding of the charge‐puddle landscape of a GNR/h‐BN structure is needed when the charge puddles function as scattering sources for mobile charge carriers. Here, a puddle landscape is constructed on the basis of an analysis of the temperature dependencies of the conductance of GNR/h‐BN devices at various gate‐voltage values. For low‐, intermediate‐, and high‐temperature regions near the charge‐neutral point (CNP), the puddle size (50–200 nm), distance between neighboring puddles (40–170 nm), and potential depth of the puddles (in a range of 10 meV) in ∼100 nm wide GNR/h‐BN devices are obtained on the basis of Coulomb blockade, 1D variable‐range hopping, and thermally activated hopping, respectively. Based on the constructed puddle landscape, it is also concluded that the confinement‐gap energy for an ∼100 nm wide GNR is similar to that of the thermal activation energy near the CNP in the GNRs. The constructed puddle landscape for GNR/h‐BN devices is consistent with that obtained from scanning tunneling microscopy observations of graphene on an h‐BN structure.

Photogating WS2 Photodetectors Using Embedded WSe2 Charge Puddles.

Performance of 2D photodetectors is often predominated by charge traps that offer an effective photogating effect. The device features an ultrahigh gain and responsivity, but at the cost of a retarded temporal response due to the nature of long-lived trap states. In this work, we devise a gain mechanism that originates from massive charge puddles formed in the type-II 2D lateral heterostructures. This concept is demonstrated using graphene-contacted WS2 photodetectors embedded with WSe2 nanodots. Upon light illumination, photoexcited carriers are separated by the built-in field at the WSe2/WS2 heterojunctions (HJs), with holes trapped in the WSe2 nanodots. The resulting WSe2 hole puddles provide a photoconductive gain, as electrons are recirculating during the lifetime of holes that remain trapped in the puddles. The WSe2/WS2 HJ photodetectors exhibit a responsivity of 3 × 102 A/W with a gain of 7 × 102 electrons per photon. Meanwhile, the zero-gate response time is reduced by 5 orders of magnitude as compared to the prior reports for the graphene-contacted pristine WS2 monolayer and WS2/MoS2 heterobilayer photodetectors due to the ultrafast intralayer excitonic dynamics in the WSe2/WS2 HJs.

Anomalous Coulomb Drag between InAs Nanowire and Graphene Heterostructures.

Correlated charge inhomogeneity breaks the electron-hole symmetry in two-dimensional (2D) bilayer heterostructures which is responsible for nonzero drag appearing at the charge neutrality point. Here we report Coulomb drag in novel drag systems consisting of a two-dimensional graphene and a one-dimensional (1D) InAs nanowire (NW) heterostructure exhibiting distinct results from 2D-2D heterostructures. For monolayer graphene (MLG)-NW heterostructures, we observe an unconventional drag resistance peak near the Dirac point due to the correlated interlayer charge puddles. The drag signal decreases monotonically with temperature (∼T^{-2}) and with the carrier density of NW (∼n_{N}^{-4}), but increases rapidly with magnetic field (∼B^{2}). These anomalous responses, together with the mismatched thermal conductivities of graphene and NWs, establish the energy drag as the responsible mechanism of Coulomb drag in MLG-NW devices. In contrast, for bilayer graphene (BLG)-NW devices the drag resistance reverses sign across the Dirac point and the magnitude of the drag signal decreases with the carrier density of the NW (∼n_{N}^{-1.5}), consistent with the momentum drag but remains almost constant with magnetic field and temperature. This deviation from the expected T^{2} arises due to the shift of the drag maximum on graphene carrier density. We also show that the Onsager reciprocity relation is observed for the BLG-NW devices but not for the MLG-NW devices. These Coulomb drag measurements in dimensionally mismatched (2D-1D) systems, hitherto not reported, will pave the future realization of correlated condensate states in novel systems.

GFET Asymmetric Transfer Response Analysis through Access Region Resistances.

Graphene-based devices are planned to augment the functionality of Si and III-V based technology in radio-frequency (RF) electronics. The expectations in designing graphene field-effect transistors (GFETs) with enhanced RF performance have attracted significant experimental efforts, mainly concentrated on achieving high mobility samples. However, little attention has been paid, so far, to the role of the access regions in these devices. Here, we analyse in detail, via numerical simulations, how the GFET transfer response is severely impacted by these regions, showing that they play a significant role in the asymmetric saturated behaviour commonly observed in GFETs. We also investigate how the modulation of the access region conductivity (i.e., by the influence of a back gate) and the presence of imperfections in the graphene layer (e.g., charge puddles) affects the transfer response. The analysis is extended to assess the application of GFETs for RF applications, by evaluating their cut-off frequency.

All-Dry Transfer of Graphene Film by van der Waals Interactions.

We report a method that uses van der Waals interactions to transfer continuous, high-quality graphene films from Ge(110) to a different substrate held by hexagonal boron nitride carriers in a clean, dry environment. The transferred films are uniform and continuous with low defect density and few charge puddles. The transfer is effective because of the weak interfacial adhesion energy between graphene and Ge. Based on the minimum strain energy required for the isolation of film, the upper limit of the interfacial adhesion energy is estimated to be 23 meV per carbon atom, which makes graphene/Ge(110) the first as-grown graphene film that has a substrate adhesion energy lower than that of typical van der Waals interactions between layered materials. Our results suggest that graphene on Ge can serve as an ideal material platform to be integrated with other material systems by a clean assembly process.

Large positive magnetoconductivity at microwave frequencies in the compensated topological insulator BiSbTeSe2

The bulk electronic properties of compensated topological insulators are strongly affected by the self-organized formation of charge puddles at low temperature, but their response in the microwave frequency range is little studied. We employed broadband impedance spectroscopy up to 5 GHz to address the ac transport properties of well-compensated BiSbTeSe2, where charge puddles are known to form as metallic entities embedded in an insulating host. It turns out that the average puddle size sets the characteristic frequency cut-off in the GHz range, across which the insulating dc behavior is separated from a metal-like high-frequency response of delocalized carriers within the puddles. The cut-off frequency is found to be controlled by a magnetic field, giving rise to a large positive magneto-conductivity observable only in the GHz range. This curious phenomenon is driven by the Zeeman energy which affects the local band filling in the disordered potential landscape to enhance the puddle size.

Charge puddles in germanene

We report an investigation of the electronic inhomogeneities in a single germanene layer grown on a molybdenum disulfide (MoS2) substrate. Using scanning tunneling microscopy and spectroscopy, we have recorded spatial maps of the Dirac point of germanene. The Dirac point maps reveal the presence of charge puddles in the germanene sheet. The Dirac point varies from −30 meV to +15 meV, corresponding to a charge density in the puddles in the range of 2.6 × 10−3 electrons to 6.6 × 10−4 holes per nm2. The radius of these puddles is about 10–20 nm, resulting in a total charge of the order of one charge carrier per puddle. The defect concentration in the top layer of the MoS2 substrate is very comparable to the density of charge puddles, suggesting that the charge puddles are caused by the charged defects in the top layer of the MoS2 substrate.

Energy relaxation of hot carriers near the charge neutrality point in HgTe-based 2D topological insulators

We present experimental results of non-linear transport in HgTe-based 2D topological insulators, where the conductance is dominated by Dirac-like helical edge states when the Fermi level is pinned to the bulk insulating gap. We find that hot carrier's energy relaxation is faster close to the charge neutrality point (CNP) which can be attributed to localized nature and incompressibility of charge puddles resulting from inhomogeneous charge distribution near CNP. The tunnel-coupling of these puddles (quantum dots) to 1D edge channels can randomize phase memory leading to incoherent inelastic processes. Hot edge carriers, excited by the electric field, relax to equilibrium via thermalization in multiple puddles resulting in the emission of phonons in the puddles. At relatively low temperature (T ≤ 10 K), the energy relaxation time shows strong temperature dependence (τε ∝ (Te−5)), which is interpreted as small angle scattering, consistent with resistance saturation at low temperatures.

Electrically tunable localized states in sub-band of bilayer graphene nanoribbon

Multiple sub-band transport in bilayer graphene nanoribbons (GNRs) with localized charge states has not been experimentally investigated owing to the difficulty of accessing into the upper sub-band. In this letter, we utilize current annealing to heavily p-dope graphene from a capping hydrogen-silsesquioxane layer. As a result, Fermi energy can be tuned into the upper sub-valence band with reasonably small gate voltage. Owing to the localized charge states, quantum dot-like characteristics are measured in bilayer GNRs, as a result of the tunnelling transport of holes in the upper sub-band through these charge puddles. In contrast to monolayer GNRs, this phenomenon appears in a considerable conductive regime since the carrier transport in the lower sub-band is still diffusive. Moreover, by electrically tuning the Fermi energy, the localized charge puddles in the upper sub-band can be resized and isolated from each other.

Unraveling Spatially Heterogeneous Ultrafast Carrier Dynamics of Single-Layer WSe2 by Femtosecond Time-Resolved Photoemission Electron Microscopy.

Studies of the ultrafast carrier dynamics of transition metal dichalcogenides have employed spatially averaged measurements, which obfuscate the rich variety of dynamics that originate from the structural heterogeneity of these materials. Here, we employ femtosecond time-resolved photoemission electron microscopy (TR-PEEM) with sub-80 nm spatial resolution to image the ultrafast subpicosecond to picosecond carrier dynamics of monolayer tungsten diselenide (WSe2). The dynamics observed following 2.41 eV pump and 3.61 eV probe occurs on two distinct time scales. The 0.1 ps process is assigned to electron cooling via intervalley scattering, whereas the picosecond dynamics is attributed to exciton-exciton annihilation. The 70 fs decay dynamics observed at negative time delay reflects electronic relaxation from the Γ point. Analysis of the TR-PEEM data furnishes the spatial distributions of the various time constants within a single WSe2 flake. The spatial heterogeneity of the lifetime maps is consistent with increased disorder along the edges of the flake and the presence of nanoscale charge puddles in the interior. Our results indicate the need to go beyond spatially averaged time-resolved measurements to understand the influence of structural heterogeneities on the elementary carrier dynamics of two-dimensional materials.

Nonuniform carrier density in Cd3As2 evidenced by optical spectroscopy

We report the detailed optical properties of Cd$_3$As$_2$ crystals in a wide parameter space: temperature, magnetic field, carrier concentration and crystal orientation. We investigate high-quality crystals synthesized by three different techniques. In all the studied samples, independently of how they were prepared and how they were treated before the optical experiments, our data indicate conspicuous fluctuations in the carrier density (up to 30%). These charge puddles have a characteristic scale of 100 $\mu$m, they become more pronounced at low temperatures, and possibly, they become enhanced by the presence of crystal twinning. The Drude response is characterized by very small scattering rates ($\sim 1$ meV) for as-grown samples. Mechanical treatment, such as cutting or polishing, influences the optical properties of single crystals, by increasing the Drude scattering rate and also modifying the high frequency optical response. Magneto-reflectivity and Kerr rotation are consistent with electron-like charge carriers and a spatially non-uniform carrier density.

Spin relaxation in disordered graphene: Interplay between puddles and defect-induced magnetism

We study the spin relaxation in graphene due to magnetic moments induced by defects. We propose and employ in our studies a microscopic model that describes magnetic impurity scattering processes mediated by charge puddles. This model incorporates the spin texture related to the defect-induced state. We calibrate our model parameters using experimentally-inferred values. The results we obtain for the spin relaxation times are in very good agreement with experimental findings. Our study leads to a comprehensive explanation for the short spin relaxation times reported in the experimental literature. We also propose a new interpretation for the puzzling experimental observation of enhanced spin relaxation times in hydrogenated graphene samples in terms of a combined effect due to disorder configurations that lead to an increased coupling to the magnetic moments and the tunability of the defect-induced π-like magnetism in graphene.

Localized Charges Control Exciton Energetics and Energy Dissipation in Doped Carbon Nanotubes.

Doping by chemical or physical means is key for the development of future semiconductor technologies. Ideally, charge carriers should be able to move freely in a homogeneous environment. Here, we report on evidence suggesting that excess carriers in electrochemically p-doped semiconducting single-wall carbon nanotubes (s-SWNTs) become localized, most likely due to poorly screened Coulomb interactions with counterions in the Helmholtz layer. A quantitative analysis of blue-shift, broadening, and asymmetry of the first exciton absorption band also reveals that doping leads to hard segmentation of s-SWNTs with intrinsic undoped segments being separated by randomly distributed charge puddles approximately 4 nm in width. Light absorption in these doped segments is associated with the formation of trions, spatially separated from neutral excitons. Acceleration of exciton decay in doped samples is governed by diffusive exciton transport to, and nonradiative decay at charge puddles within 3.2 ps in moderately doped s-SWNTs. The results suggest that conventional band-filling in s-SWNTs breaks down due to inhomogeneous electrochemical doping.

Evaluating the Sources of Graphene\u2019s Resistivity Using Differential Conductance

We explore the contributions to the electrical resistance of monolayer and bilayer graphene, revealing transitions between different regimes of charge carrier scattering. In monolayer graphene at low densities, a nonmonotonic variation of the resistance is observed as a function of temperature. Such behaviour is consistent with the influence of scattering from screened Coulomb impurities. At higher densities, the resistance instead varies in a manner consistent with the influence of scattering from acoustic and optical phonons. The crossover from phonon-, to charged-impurity, limited conduction occurs once the concentration of gate-induced carriers is reduced below that of the residual carriers. In bilayer graphene, the resistance exhibits a monotonic decrease with increasing temperature for all densities, with the importance of short-range impurity scattering resulting in a “universal” density-independent (scaled) conductivity at high densities. At lower densities, the conductivity deviates from this universal curve, pointing to the importance of thermal activation of carriers out of charge puddles. These various assignments, in both systems, are made possible by an approach of “differential-conductance mapping”, which allows us to suppress quantum corrections to reveal the underlying mechanisms governing the resistivity.

Enhanced electron dephasing in three-dimensional topological insulators

Study of the dephasing in electronic systems is not only important for probing the nature of their ground states, but also crucial to harnessing the quantum coherence for information processing. In contrast to well-studied conventional metals and semiconductors, it remains unclear which mechanism is mainly responsible for electron dephasing in three-dimensional topological insulators (TIs). Here, we report on using weak antilocalization effect to measure the dephasing rates in highly tunable (Bi,Sb)2Te3 thin films. As the transport is varied from a bulk-conducting regime to surface-dominant transport, the dephasing rate is observed to evolve from a linear temperature dependence to a sublinear power-law dependence. Although the former is consistent with the Nyquist electron-electron interactions commonly seen in ordinary 2D systems, the latter leads to enhanced electron dephasing at low temperatures and is attributed to the coupling between the surface states and the localized charge puddles in the bulk of 3D TIs.

Anderson Insulators in Self-Assembled Gold Nanoparticles Thin Films: Single Electron Hopping between Charge Puddles Originated from Disorder.

The Anderson insulating states in Au nanoparticle assembly are identified and studied under the application of magnetic fields and gate voltages. When the inter-nanoparticle tunneling resistance is smaller than the quantum resistance, the system showing zero Mott gap can be insulating at very low temperature. In contrast to Mott insulators, Anderson insulators exhibit great negative magnetoresistance, inferring charge delocalization in a strong magnetic field. When probed by the electrodes spaced by ~200 nm, they also exhibit interesting gate-modulated current similar to the multi-dot single electron transistors. These results reveal the formation of charge puddles due to the interplay of disorder and quantum interference at low temperatures.