Role Of Surface States Research Articles

Features of current flow in the n-CoFe2O4/n-CdTe heterojunction

n-CoFe2O4/n-CdTe heterojunctions with a current rectification ratio of 3·105 at voltages |V| = 1.5 V were made by spray pyrolysis of aqueous solutions of cobalt and iron salts on n-CdTe substrates. Based on the analysis of the temperature dependence of I-V-characteristics in the forward voltage range, a change in the mechanisms of current flow in n-CoFe2O4/n-CdTe heterojunctions was established from overbarrier at voltages of 3kT/q < V < 0.3 V to tunneling at voltages of 0.4 V < V < 1 V. The role of surface states in the formation of the energy profile of the heterojunction and the participation of energy levels in the band gap of n-CdTe in the formation of the tunnel current have been clarified. The reasons for the occurrence of negative differential resistance at the forward biases of the structure have been clarified. The current at reverse biases in the range −3 V < V < -3kT/q was analyzed. According to the analysis of the C-V-characteristics, an inversion layer was found in the heterojunction and its behavior from voltage was explained.

Origin of linear magnetoresistance in Bi2Te3 topological insulator: Role of surface state and defects

Connection between topological surface states (TSS) and large linear magnetoresistance (LMR) is established in tetradymite Bi2Te3 through tuning of native defects. Thorough analysis of temperature dependent synchrotron X-ray diffraction data quantifies 0-D, 1-D and 2-D defect concentrations. Plausible variation of antisite defects is explained via carrier concentration data. Resistivity data divulge contribution of mixed bulk and surface states in the synthesized samples. High Fermi velocity (∼106 ms−1) and low Fermi energy (∼14 meV) of carriers are obtained. Lower defect concentration sample emerges with high magnetoresistance (MR) and higher mobility. Weak antilocalization (WAL) effect, signature of TSS, is revealed from MR. Large (80%), non-saturating LMR along with ultrahigh mobility (∼1 m2V−1s−1) in Bi2Te3 supports the presence of TSS. Magnetoconductance data are fitted with Hikami-Larkin-Nagaoka equation, obtaining further insight on WAL effect. In addition, extracted parameters like disorder correlation length and coherence length explicate the role of defect density on LMR.

Surface-Mediated Charge Transfer of Photogenerated Carriers in Diamond.

Solvated electrons are highly reductive chemical species whose chemical properties remain largely unknown. Diamond materials are proposed as a promising emitter of solvated electrons and visible light excitation would enable solar-driven CO2 or N2 reductions reactions in aqueous medium. But sub-bandgap excitation remains challenging. In this work, the role of surface states on diamond materials for charge separation and emission in both gaseous and aqueous environments from deep UV to visible light excitation is elucidated. Four different X-ray and UV-vis spectroscopy methods are applied to diamond materials with different surface termination, doping and crystallinity. Surface states are found to dominate sub-bandgap charge transfer. However, the surface charge separation is drastically reduced for boron-doped diamond due to a very high density of bulk defects. In a gaseous atmosphere, the oxidized diamond surface maintains a negative electron affinity, allowing charge emission, due to remaining hydrogenated and hydroxylated groups. In an aqueous electrolyte, a photocurrent for illumination down to 3.5eV is observed for boron-doped nanostructured diamond, independent of the surface termination. This study opens new perspectives on photo-induced interfacial charge transfer processes from metal-free semiconductors such as diamonds.

Roles of Surface States in Cocatalyst-Loaded Hematite Photoanodes for Water Oxidation

Photoelectrochemical water oxidation over bare hematite (α-Fe2O3) involved physical–chemical processes through surface states. It is found that the surface state with a higher oxidative energy (S1) served as a reaction intermediate for water oxidation, while the other surface state with a lower oxidative energy (S2) was basically catalytically inactive and induced charge recombination. Since a cocatalyst is commonly loaded on α-Fe2O3 to enhance water oxidation, the physical–chemical processes between the surface states and the cocatalyst are supposedly important but remain unclear. In the present study, we elucidated such processes by employing photoelectrochemical impedance spectroscopy. We found that, in a CoPi-loaded α-Fe2O3, S1 no longer served as an intermediate for water oxidation and was mostly passivated. At the same time, S2 became the key intermediate for water oxidation by serving as a hole reservoir, prolonging the hole lifetime, and finally transferring holes to CoPi. This study reveals the surface chemistry for optimizing cocatalyst-loaded α-Fe2O3, which is different from that for bare α-Fe2O3.

The role of surface states and point defects on optical properties of InGaN/GaN multi-quantum wells in nanowires grown by molecular beam epitaxy

The optical properties of nanowire-based InGaN/GaN multiple quantum wells (MQWs) heterostructures grown by plasma-assisted molecular beam epitaxy are investigated. The beneficial effect of an InGaN underlayer grown below the active region is demonstrated and assigned to the trapping of point defects transferred from the pseudo-template to the active region. The influence of surface recombination is also investigated. For low InN molar fraction value, we demonstrate that AlO x deposition efficiently passivate the surface. By contrast, for large InN molar fraction, the increase of volume non-radiative recombination, which we assign to the formation of additional point defects during the growth of the heterostructure dominates surface recombination. The inhomogeneous luminescence of single nanowires at the nanoscale, namely a luminescent ring surrounding a less luminescent centre part points towards an inhomogeneous spatial distribution of the non-radiative recombination center tentatively identified as intrinsic point defects created during the MQWs growth. These results can contribute to improve the performances of microLEDs in the visible range.

Unraveling the surface states related Stokes shift dependent electrocatalytic activity of N-doped carbon quantum dots for photovoltaic applications

Modulating the optical properties of carbon quantum dots (CQDs), such as blue to red emission, by varying the surface states is widely used for optical devices. However, the role of surface states in the electrocatalytic activity of CQDs remains incompletely elucidated. In this regard, we modulated the surface states by varying the synthesis solvents to obtain CQDs with varying Stokes shift: blue-, green-, and red-emissive CQDs denoted as BDs, GDs, and RDs, respectively. The electrocatalytic activity of these CQDs were investigated. To prevent aggregation-induced quenching of the electrocatalytic activity and enhance the conductivity, CQDs were grown on carbon nanotubes (CNT), which provide multidimensional charge transport channels. The RDs showed the largest Stokes-shift and better electrocatalytic activity than those of the GDs and BDs. The underlying mechanism for the superior electrocatalytic activity of the RDs was related to their effective band tuning, enhanced surface reactivity, and fast charge transfer. The performance of dye-sensitized solar cells employing RDs-modified CNT as counter electrode is comparable to that of a conventional Pt-based counter electrode.

Enhancing the UV Emission in ZnO-CNT Hybrid Nanostructures via the Surface Plasmon Resonance of Ag Nanoparticles.

The crystal quality and surface states are two major factors that determine optical properties of ZnO nanoparticles (NPs) synthesized through nonaqueous sol–gel routes, and both are strongly dependent on the growth conditions. In this work, we investigate the influence of the different growth temperatures (240 and 300 °C) on the morphology, structural and crystal properties of ZnO NP. The effects of conjoining ZnO NP with carbon nanotubes (CNT) and the role of surface states in such a hybrid nanostructure are studied by optical emission and absorption spectroscopy. We demonstrate that depending on the synthesis conditions, activation or passivation of certain surface states may occur. Next, silver nanoparticles are incorporated into ZnO–CNT nanostructures to explore the plasmon–exciton coupling effect. The observed enhanced excitonic and suppressed defect-related emissions along with blue-shifted optical band gap suggest an intricate interaction of Burstein–Moss, surface plasmon resonance and surface band-bending effects behind the optical phenomena in hybrid ZnO–CNT–Ag nanocomposites.

Dynamical imaging of local photovoltage at semiconductor surface by photo-assisted ultrafast scanning electron microscopy

Photo-assisted Ultrafast Scanning Electron Microscopy (USEM) maps the dynamics of surface photovoltages and local electric fields in semiconducting samples. Photovoltages and their gradients close to surface affect the emission yield and the detection efficiency of secondary electrons (SE), leading to photoexcited SE 2D patterns. In this work, we present a method to characterize the evolution of the patterns up to ultrafast regime. These results reveal the role of surface states in affecting the external field dynamics at picoseconds. Moreover, we show that tiny changes in surface preparation express deeply different photoexcited voltage signals. We investigate the relation between the surface chemistry of Si and photo-induced SE contrast.

High Spin to Charge Conversion Efficiency in Electron Beam-Evaporated Topological Insulator Bi2Se3.

Bi2Se3 is a well-established topological insulator (TI) having spin momentum locked Dirac surface states at room temperature and predicted to exhibit high spin to charge conversion efficiency (SCCE) for spintronics applications. The SCCE in TIs is characterized by an inverse Edelstein effect length (λIREE). We report an λIREE of ∼0.36 nm, which is the highest ever observed in Bi2Se3. Here, we performed spin pumping and inverse spin Hall effect (ISHE) in an electron beam-evaporated Bi2Se3/CoFeB bilayer. The Bi2Se3 thickness dependence of λIREE, perpendicular surface anisotropy (KS), spin mixing conductance, and spin Hall angle confirmed that spin to charge conversion is due to spin momentum locked Dirac surface states. We propose that the role of surface states in SCCE can be understood by the evaluation of KS. The SCCE is found to be high when the value of KS is small.

Role of surface states on luminescence shift of caramelised sugar carbon dots for color conversion emitting devices

Carbon dots made of various sources have shown unique and excellent optical properties for many potential applications, including light emitting devices. The luminescence origin of carbon dots is crucial in colour conversion emitting device applications. In this work, an intensive study on surface state effects of caramelised sugar carbon dots for colour conversion applications was done. Using caramelised carbon dots synthesised by microwave-assisted technique, we studied optical properties of carbon dots using photoluminescence, time resolved photoluminescence, Fourier-transform infrared spectroscopies, absorbance spectroscopy and Raman spectroscopy. We found that different concentrations of caramelised sugar dissolved in water caused absorbance and emission peak shift of carbon dots. However, life-time, crystal structure and functional groups of carbon dots were not significantly affected by carbon dot concentration. We have utilised this shifting luminescence effect in colour conversion light emitting devices using blue and green light emitting diodes. Caramelised sugar carbon dots, which were placed on top of light emitting diodes, were able to generate new colours. This work is useful for developing advanced lighting devices using carbon dots.

(Invited) Role of Surface States in Photocatalytic Oxygen Evolution with CuWO4 Particles

CuWO4 is a medium bandgap (2.3 eV) n-type semiconductor capable of photoelectrochemical water oxidation under applied electrical bias. Here, we show for the first time that suspended microcrystals CuWO4 evolve oxygen photocatalytically under visible illumination from solutions of 0.05 M AgNO3(10.8 μmol/hour; AQE of 0.561% at 400 nm) and 0.0002 M FeCl3(1.5 μmol/hour). No oxygen is detected with 0.002 M [Fe(CN)6]3-as sacrificial agent. The activity dependence on the redox potential of the acceptors is due to the presence of Cu2+based electron trap states in CuWO4. According to surface photovoltage spectroscopy and electrochemistry, these states are located on the particle surface, 1.8 eV above the valence band edge of the material. Controlling the chemistry of these states will be key to uses of CuWO4particles in tandem catalysts for overall water splitting. Figure 1

Role of Surface States in Photocatalytic Oxygen Evolution with CuWO4 Particles

CuWO4 is a medium bandgap (2.3 eV) n-type semiconductor capable of photoelectrochemical water oxidation under applied electrical bias. Here, we show for the first time that suspended microcrystals CuWO4 evolve oxygen photocatalytically under visible illumination from solutions of 0.05 M AgNO3 (10.8 μmol/hour; AQE of 0.56% at 400 nm) and 0.0002 M FeCl3 (1.5 μmol/hour). No oxygen is detected with 0.002 M [Fe(CN)6]3− as sacrificial agent. The activity dependence on the redox potential of the acceptors is due to the presence of Cu2+ based electron trap states in CuWO4. According to surface photovoltage spectroscopy and electrochemistry, these states are located on the particle surface, 1.8 eV above the valence band edge of the material. Controlling the chemistry of these states will be key to uses of CuWO4 particles in tandem catalysts for overall water splitting.

Variable range hopping conduction in ZnO nanocrystal thin films

Zinc oxide (ZnO) nanocrystal films are of interest for new applications in thin film transistors and as transparent conductive oxides. Previous work has concentrated on achieving highly conductive, metallic films. This work focusses on the less explored insulating to semi-insulating regime, which enables obtaining deeper insights into the roles of surface states and defect states trapped at the nanocrystal interfaces. We examine the effects of various post-deposition treatments including controlled dosing with ultraviolet light, filling the voids between nanocrystals with a matrix material deposited by atomic layer deposition, and thermal annealing of the nanocrystal films. Both Mott and Efros–Shklovskii variable range hopping are observed depending on the carrier concentration in the nanocrystals. Using the above post-treatments to transition the films between the two conduction mechanisms enables determining the Fermi level density of states and the electron localization length. To interpret our results, we propose a model based on the assumption of nanocrystals consisting of quasi-neutral cores surrounded by shells depleted by surface OH trap states. The model suggests that the primary source of the increased conductivity in ZnO nanocrystal films based on post-treatments is an increase in the ability to tunnel between nanocrystals due to a reduction of the distance between the quasi-neutral nanocrystal cores.

Role of Surface States and Interface Charges in 2DEG in Sputtered ZnO Heterostructures

In this paper, we report on the influence of surface (donor and acceptor) states and interface charges on the formation of 2-D electron gas (2DEG) in MgZnO/ZnO heterostructure grown by sputtering. Donor and acceptor states alone cannot yield an order of magnitude higher value of 2DEG as observed by sputtering growth as compared to epitaxial techniques. While surface donor states are reported to govern the formation of 2DEG in epitaxially grown heterostructures, our simulations suggest a non-negligible contribution of surface acceptor states in the 2DEG density. In addition, the existence of interface charges due to high defect density along with appropriate values of donor and acceptor states is essential to accurately predict an order of magnitude higher 2DEG density grown by sputtering. This paper analyzes the distinct roles of donor and acceptor states along with interface charges in 2DEG formation and achieved values.

The role of surface states during photocurrent switching: Intensity modulated photocurrent spectroscopy analysis of BiVO4 photoelectrodes

Intriguing photo-electrochemical characteristics of BiVO4 photoelectrodes studied in pH-neutral aqueous solutions are reported herein. Indeed, we have observed photocurrent polarity switching, as put in evidence by cyclic voltammetry under chopped illumination conditions. Such unusual behavior was analyzed in detail using Intensity Modulated Photocurrent Spectroscopy (IMPS). At potentials where positive photocurrent was observed, the expected shape of IMPS was recorded, starting in quadrant (IV) at high-frequency (HF) and reaching quadrant (I) at low-frequency (LF) with two well defined semicircles. Surprisingly, in the negative photocurrent region, IMPS started in quadrant (II) at HF and ended in quadrant (III) at LF. Such highly infrequent features were interpreted here as the rotation of the IMPS spectra around the origin of the plot, due to the sign switch of the photocurrent. A model that takes into account the existence of in-band energy states at the surface of BiVO4 has been used in order to account for the experimental results. It was found that (i) the surface state capacitance; (ii) the relaxation time constant associated to surface states; and (iii) the density of in-gap surface states, were all showing a well-marked maximum in the nearby value of the switch potential. This suggests that surface states are more influent in the nearby where photocurrent switch occurs.

Role of Surface States in Silver-Doped CdSe and CdSe/CdS Quantum Dots

The effects of Ag doping of CdSe and CdSe/CdS quantum dots (QDs) have been studied as a function of the surface composition. Throughout these studies, static and time-resolved luminescence and transient absorption measurements are used to determine the nature and rates of radiationless decay mechanisms and how these vary with the concentration of Ag dopants. In tributylphosphine-ligated CdSe, the photoluminescence quantum yield (PL QY) varies nonmonotonically with dopant concentration, increasing at low concentrations of Ag+ then decreasing with further addition. The initial increase is assigned to the passivation of preexisting surface hole traps by interaction of interstitial Ag+ with surface Se2– ions, whereas the subsequent decrease is due to the introduction of substitutional Ag+ dopants, which act as a new source of hole traps. Ligation of the particle surface with alkyl amines largely passivates the surface hole traps and thereby eliminates the initial increase in PL QY upon doping. CdSe/CdS QDs li...

Role of Tungsten Doping on the Surface States in BiVO4 Photoanodes for Water Oxidation: Tuning the Electron Trapping Process

The nanostructured BiVO4 photoanodes were prepared by electrospinning and were further characterized by XRD, SEM, and XPS, confirming the bulk and surface modification of the electrodes attained by W addition. The role of surface states (SS) during water oxidation for the as-prepared photoanodes was investigated by using electrochemical, photoelectrochemical, and impedance spectroscopy measurements. An optimum 2% doping is observed in voltammetric measurements with the highest photocurrent density at 1.23 VRHE under back side illumination. It has been found that a high PEC performance requires an optimum ratio of density of surface states (NSS) with respect to the charge donor density (Nd), to give both good conductivity and enough surface reactive sites. The optimum doping (2%) shows the highest Nd and SS concentration, which leads to the high film conductivity and reactive sites. The reason for SS acting as reaction sites (i-SS) is suggested to be the reversible redox process of V5+/V4+ in semiconductor...

In2O3-Based Thermoelectric Materials: The State of the Art and the Role of Surface State in the Improvement of the Efficiency of Thermoelectric Conversion

In this paper, the thermoelectric properties of In2O3-based materials in comparison with other thermoelectric materials are considered. It is shown that nanostructured In2O3 Sn-based oxides are promising for thermoelectric applications at moderate temperatures. Due to the nanostructure, specific surface properties of In2O3 and filtering effects, it is possible to significantly reduce the thermal conductivity and achieve an efficiency of thermoelectric conversion inaccessible to bulk materials. It is also shown that a specific surface state at the intergrain boundary, optimal for maximizing the filtering effect, can be achieved through (1) the engineering of grain boundary parameters, (2) controlling the composition of the surrounding atmosphere, and (3) selecting the appropriate operating temperature.

The Role of Surface States in the Electrochemical Behavior of Semiconductor-Electrolyte Interfaces from First Principles

Solar energy is the most abundant energy source available to humankind; however, this energy cannot be harnessed on demand due to the variability of sunlight. Artificial photosynthesis offers a sustainable way to overcome that variability through the photocatalytic conversion of solar power into chemical fuels at a semiconductor–electrolyte interface. Although considerable progress has been made in simulating the bulk properties of semiconductors from first principles, much less has been done to address the electrochemical response of electrified semiconductor–electrolyte interfaces under realistic environmental conditions. In particular, band bending and surface states are two important contributors to the photogeneration of electrical current at semiconductor–electrolyte interface. We present a broadly applicable and highly transferable computational techniques to simulate semiconductor–electrolyte interfaces. By introducing a continuum model of the semiconductor and electrolyte regions surrounding the quantum-mechanical interfacial layer, the model enables for the self-consistent determination of the surface states and of the Schottky barriers that subsequently form within the space charge of the semiconductor electrodes. Validation of these simulations is provided by the analysis of experimental charge-voltage measurements, focusing on the photovoltage region close to the onset of the photocurrent, which depends strongly on the properties of surface states. Our model provides a detailed understanding of the electrochemical stability and intrinsic current–voltage response of semiconductor electrodes taking into account the surface states that arise from the adsorption of oxygen and hydrogen species towards optimizing photocatalytic performance.

Photoluminescence in Functionalized/Doped Graphene Quantum Dots: Role of Surface States

Photoluminescence in Functionalized/Doped Graphene Quantum Dots: Role of Surface States It has been shown that functionalization/doping of Graphene Quantum Dots (GQDs) can lead to further tuning of their already existing band gap. We have successfully functionalized GQDs (synthesized by hydrothermal process) with oxygen, hydrogen, fluorine, and doped with nitrogen using capacitively coupled plasma of respective gas as evidenced by Raman and X-ray photo emission spectroscopy (XPS). Room temperature photoluminescence (PL) of each functionalized GQD shows distinctive features and can be explained as due to charge transfers between the functional groups/dopants and GQDs as well as presence of mid gap states as a result of functionalization and doping. An energy diagram model is proposed to elucidate the PL modulation resulting from functionalization and doping.