Change In Band Bending Research Articles

Transfer doping of epitaxial graphene on SiC(0001) using Cs

Control of the charge carrier concentration is essential for applications of graphene. Here, we demonstrate the doping of epitaxial graphene on SiC(0001) via charge transfer from an adsorbed layer of Cs atoms with sub-monolayer coverage. The electronic structure of the graphene is analyzed using x-ray and angle-resolved photoelectron spectroscopy. In H-intercalated, quasi-freestanding monolayer graphene (QFMLG), the Dirac point can be tuned continuously from p-type to strong n-type doping. For strong n-type doping, analysis of the core level binding energies implies a deviation from a rigid band shift. This might be explained by an increased screening of the atomic core potential due to the higher number of charge carriers per C atom in the graphene layer. Furthermore, charge transfer into the SiC substrate leads to a change in band bending at the SiC/QFMLG interface, which saturates into a flat band scenario at higher Cs coverage. An analysis of the Fermi surfaces suggests an increasing electron-phonon-coupling in strongly doped QFMLG. In monolayer graphene (MLG), which is intrinsically n-type doped due to the presence of the buffer layer at the SiC interface, n-type doping can be enhanced by Cs evaporation in a similar fashion. In contrast to QFMLG, core level spectra and Dirac cone position in MLG apparently show a rigid band shift even for very high doping, emphasizing the importance of the substrate.

Leveraging plasmonic hot electrons to quench defect emission in metal-semiconductor nanostructured hybrids.

Modeling light-matter interactions in hybrid plasmonic materials is vital to their widening relevance from optoelectronics to photocatalysis. Here, we explore photoluminescence (PL) from ZnO nanorods (ZNRs) embedded with gold nanoparticles (Au NPs). A progressive increase in Au NP concentration introduces significant structural disorder and defects in ZNRs, which paradoxically quenches defect related visible PL while intensifying the near band edge (NBE) emission. Under UV excitation, the simulated semi-classical model realizes PL from ZnO with sub-bandgap defect states, eliciting visible emissions that are absorbed by Au NPs to generate a non-equilibrium hot carrier distribution. The photo-stimulated hot carriers, transferred to ZnO, substantially modify its steady-state luminescence, reducing NBE emission lifetime and altering the abundance of ionized defect states, finally reducing visible emission. The simulations show that the change in the interfacial band bending at the Au-ZnO interface under optical illumination facilitates charge transfer between the components. This work provides a general foundation to observe and model the hot carrier dynamics and strong light-matter interactions in hybrid plasmonic systems.

Application of sensor structures based on a photoelectric transducer to determine the activity of aspartate and alanine aminotransferases in blood plasma

The successful application of a recombination sensor for the real-time detection of transaminasethe detection of transaminase activities (ALT/AST) in the blood plasma of rats has been demonstrated. The parameter directly measured in real time is the photocurrent through the structure with buried silicon barrier when light with high absorption coefficient is used. Detection is realized as a result of specific chemical reactions catalyzed by ALT and AST enzymes (α-ketoglutarate + aspartate and α-ketoglutarate + alanine). The change of the effective charge of the reagents allows recording the activity of enzymes from photocurrent measurements. The main factor in this approach is the influence on the parameters of the recombination centers at the interface. The physical mechanism of the sensor structure can be explained within the framework of the Stevenson theory, taking into account the changes in the pre surface band bending, the capture cross sections and the energy position of the recombination levels during adsorption. The paper also offers theoretical analyze allowing optimization of analytical signals of recombination sensor. A promising approach to develop a simple and sensitive method for real time detection of transaminases activity has been discussed in detail.

The Role of Electrolytes in the Relaxation of Near-Surface Spin Defects in Diamond.

Quantum sensing with spin defects in diamond, such as the nitrogen vacancy (NV) center, enables the detection of various chemical species on the nanoscale. Molecules or ions with unpaired electronic spins are typically probed by their influence on the NV center's spin relaxation. Whereas it is well-known that paramagnetic ions reduce the NV center's relaxation time (T1), here we report on the opposite effect for diamagnetic ions. We demonstrate that millimolar concentrations of aqueous diamagnetic electrolyte solutions increase the T1 time of near-surface NV center ensembles compared to pure water. To elucidate the underlying mechanism of this surprising effect, single and double quantum NV experiments are performed, which indicate a reduction of magnetic and electric noise in the presence of diamagnetic electrolytes. In combination with ab initio simulations, we propose that a change in the interfacial band bending due to the formation of an electric double layer leads to a stabilization of fluctuating charges at the interface of an oxidized diamond. This work not only helps to understand noise sources in quantum systems but could also broaden the application space of quantum sensors toward electrolyte sensing in cell biology, neuroscience, and electrochemistry.

In Situ Observation of Photoinduced Halide Segregation in Mixed Halide Perovskite

Mixed–halide perovskites have emerged as a promising candidate for optoelectronics, due to their tunable optical properties. However, photoinduced phase segregation remains an obstacle for stable performance in solar cells. Here, we have conducted both bulk and nanoscale measurements to elucidate the mechanism of halide ion migration that leads to phase segregation. By utilizing Kelvin probe force microscopy (KPFM) under illumination, we have observed the time-evolution of ion migration to and from grain boundaries that agreed with bulk photoluminescence spectra of perovskites with a wide range of band gaps (1.67–1.88 eV), Cs0.15FA0.65MA0.20Pb(IxBr1–x)3 where x = 0.47–0.80. By visualizing the changes of band bending at grain boundaries, we deduce that halide segregation is dominantly caused by iodide ions, given faster ion migration in perovskite materials with a higher iodine content. Further, we verify that the changing rate of band bending at grain boundaries is consistent with the emerging rate of I-rich phase at grain boundaries, suggesting the influence of iodide ion migration toward grain boundaries on I-rich phase transition. This work will help provide insight for interpreting the mechanism of light-halide ion interactions.

Influence of environmental conditions and surface treatments on the photoluminescence properties of GaN nanowires and nanofins

Due to their intrinsically large surface-to-volume ratio, nanowires and nanofins interact strongly with their environment. We investigate the role of the main air constituents nitrogen, oxygen and water on the efficiency of radiative recombination in GaN nanostructures as a function of different surface treatments and at temperatures up to 200 °C. Oxygen and water exposures exhibit a complex behavior as they can both act quenching and enhancing on the photoluminescence intensity dependent on the temperature. For oxygen, these characteristics are already observed for low concentrations of below 0.5% in nitrogen. While the photoluminescence intensity changes induced by oxygen occur independently of illumination, the influence of water is light-induced: it evolves within tens of seconds under ultraviolet light exposure and is heavily influenced by the nanostructure pre-treatment. In contrast to observations in dry atmospheres, water prevents a recovery of the photoluminescence intensity in the dark. Combined measurements of the electrical current through GaN nanofins and their photoluminescence intensity reveal the environmental influence on the interaction of non-radiative recombination processes and changes in the surface band bending of the nanostructures. Several investigated solvents show an enhancing effect on the PL intensity increase, peaking in c-hexane with a 26-fold increase after 6 min of light exposure. Stabilization of the PL intensity was achieved by a passivation of the GaN surface with Ga x O y , and ZnO shells. Surprisingly, Al2O3 coatings resulted in a highly instable PL intensity during the first minutes of illumination. Our findings reveal the high importance of controlled environmental conditions for the investigation of nanostructures, especially when aimed at their applications in the fields of environmental sensing, photo-catalysis and light-emitting diodes.

Chemoresistive Gas Sensors Based on SnO2 and Sn3O4 Nanobelts to Volatile Organic Compounds Detection: A Comparative Investigation

Introduction Volatile organic compounds (VOCs) are organic chemicals with a relatively high vapor pressure at low temperatures released to the environment from biogenic and anthropogenic sources. Hundreds of VOCs have been identified in the human breath, and many of them act as potential biomarkers for noninvasive diagnosis and monitoring of several diseases [1]. Contrarily, the increasing emission of these hazardous compounds in urban areas, mainly from industry and automotive exhaust systems, has been a critical matter of modern society in terms of safety, environmental pollution, and public health [2].Chemoresistors based on semiconducting metal oxides (SMOx) offer unique opportunities for trace detection of VOCs, considering their compatibility for portable devices, simple design, low-cost, and reduced power consumption [3]. While SnO2 has been the most used material in the active layer of market sensors, n-type Sn3O4 is another possible stoichiometry of tin oxide with emergent interests in the gas sensing field over the last years [4,5]. However, only limited research has reported the response of Sn3O4 nanostructures to VOCs detection.In this work, the sensing properties of single-crystalline Sn3O4 nanobelts towards some relevant volatile organic compounds (ethanol, acetone, and toluene) were investigated comparatively to SnO2 in multiple operating conditions. A comprehensive understanding of the conduction mechanism associated with the performance of the sensors was additionally discussed. Materials and Methods Sn3O4 and SnO2 nanobelts were synthesized in a tube furnace by the carbothermal reduction method following optimized synthesis parameters described in our previous work [4]. The materials were collected from different regions of the growth site and separately dispersed in 1,2-propanediol by sonication. Several drops of the solutions were deposited onto alumina substrates containing interdigitated Pt electrodes on the topside and a Pt heater on the backside. The sensors were dried at 80 °C overnight and then annealed at 350°C for 10 minutes.The devices were inserted into a homemade measurement chamber coupled with a gas mixing system equipped with mass-flow controllers. Several concentrations of the target gases, including ethanol, acetone, and toluene, were introduced into the chamber, and the DC electrical resistances were continually monitored at 200, 250, and 300°C. Synthetic air in dry and humid conditions (5%, 30%, and 70% r.h. at 25°C) was used as the reference gas. A constant gas flow of 200 sccm was maintained during all the experiments. Results and Conclusions The gas sensing measurements performed from 200 to 300°C revealed that the baseline resistance of SnO2 and Sn3O4 sensors changes accordingly with the humidity level in the background. It occurs due to the reducing nature of the water vapor on the surface of tin oxide. After exposure to ethanol, acetone, and toluene, the resistance of both sensors decreases following the gas concentrations. These reducing gas species act as surface donors, which improves the number of free electrons in n-type semiconductors. From the calibration curves, the signals of the SnO2 sensor toward toluene, acetone, and ethanol drop as a result of the increment in the relative humidity over the monitored operating temperatures. The same profile was observed in the Sn3O4 sensors at 200°C and 250°C, but the opposite outcome takes place at 300°C, i.e., higher sensor signals were achieved in humid conditions for the tested VOCs. In addition, the sensors exhibit remarkable selectivity to potential interferents such as H2 and CO in the full range of concentrations and working temperature investigated. Changes in the surface band bending were correlated with electrical resistance data based on a depletion layer model. It allowed obtaining information about the electronic structure as a function of the charge carrier concentration induced by the analytes chemisorption on the sensing layers. To the best of our knowledge, this is the first report on 1D single-crystalline Sn3O4 materials towards VOCs detection, opening up new possibilities for further investigations. References Chan, L.W.; Anahtar, M.N.; Ong, T.H.; Hern, K.E.; Kunz, R.R.; Bhatia, S.N. Engineering synthetic breath biomarkers for respiratory disease. Nat. Nanotechnol. 2020, 15, 792–800. He, C.; Cheng, J.; Zhang, X.; Douthwaite, M.; Pattisson, S.; Hao, Z. Recent advances in the catalytic oxidation of volatile organic compounds: A review based on pollutant sorts and sources. Chem. Rev. 2019, 119, 4471–4568. Lin, T.; Lv, X.; Hu, Z.; Xu, A.; Feng, C. Semiconductor metal oxides as chemoresistive sensors for detecting volatile organic compounds. Sensors (Switzerland) 2019, 19, 233. Suman, P.H.; Longo, E.; Varela, J.A.; Orlandi, M.O. Controlled synthesis of layered Sn3O4 nanobelts by carbothermal reduction method and their gas sensor properties. J. Nanosci. Nanotechnol. 2014, 14, 6662–6668. Suman, P.H.; Felix, A.A.; Tuller, H.L.; Varela, J.A.; Orlandi, M.O. Comparative gas sensor response of SnO2, SnO and Sn3O4 nanobelts to NO2 and potential interferents. Sensors Actuators, B Chem. 2015, 208, 122–127.

Electroreduction of Aryldiazonium Ion at the Polar and Non-Polar Faces of ZnO: Characterisation of the Grafted Films and Their Influence on Near-Surface Band Bending.

ZnO is a strong candidate for transparent electronic devices due to its wide band gap and earth-abundance, yet its practical use is limited by its surface metallicity arising from a surface electron accumulation layer (SEAL). The SEAL forms by hydroxylation of the surface under normal atmospheric conditions, and is present at all crystal faces of ZnO, although with differing hydroxyl structures. Multilayer aryl films grafted from aryldiazonium salts have previously been shown to decrease the downward bending at O-polar ZnO thin films, with Zn-O-C bonds anchoring the aryl films to the substrate. Herein we show that the Zn-polar (0001), O-polar (000 ), and non-polar m-plane (10 0) faces of ZnO single crystals, can also be successfully electrografted with nitrophenyl (NP) films. In all cases, X-ray photoelectron spectroscopy (XPS) measurements reveal that the downward surface band bending decreases after modification. XPS provides strong evidence for Zn-O-C bonding at each face. Electrochemical reduction of NP films on O-polar ZnO single crystals converts the film to a mainly aminophenyl layer, although with negligible further change in band bending. This contrasts with the large upward shifts in band bending caused by X-ray induced reduction.

UV-induced change in channel conductivity in AlGaN/GaN high electron mobility transistors to measure doping

AlGaN/GaN High Electron Mobility Transistors (HEMTs) frequently show an UV-induced increase in channel conductivity (persistent photoconductivity—PPC). PPC is a generic effect that exists for a wide range of HEMT configurations irrespective of the substrate material or the nominal buffer doping type and magnitude. Simulations demonstrate that the magnitude of the PPC effect can be explained by the UV-induced change in electronic band bending in the buffer below the 2D electron gas (2DEG). It only has a significant magnitude for p-type buffers that result from the presence of intentional or unintentional carbon doping and allows the net deep-level doping density (i.e., |NA-ND|) below the 2DEG to be quantified, a parameter that is extremely difficult to evaluate by any other means. Transient measurements indicate a PPC recovery time extending to several days, with leakage along dislocations playing an important role in this recovery.

Injection Metal-Assisted Catalytic Etching (MACE) of Si Powder: Discovery of Low-Load MACE and Pore Distribution Tunability Using Ag, Au, Pd, Pt and Cu Catalysts

Injection of H2O2 to control both the rate and extent of etching allowed us to discover a new regime of MACE with extremely small quantities of deposited metal as a catalyst: low-load MACE (LL-MACE).1 The structure of particles subjected to LL-MACE is completely different compared to conventional MACE. Si nanowires and mesoporous micro- or nano-particles are produced with high yield, low cost and controlled properties suitable for applications in e.g. lithium-ion batteries, drug delivery, as well as biomedical imaging and contrast enhancement. Different metal catalysts behave in distinct manners than can be traced to differences in nucleation and reaction kinetics.2 Not only Ag, Au, Pd and Pt but also inexpensive Cu can be used to perform LL-MACE.Previously, controlled reactant injection and acetic acid were used to transform stain etching into the improved process known as regenerative electroless etching (ReEtching).3 Here injection and acetic acid are exploited to enhance MACE and extend it to the low-load regime. Ag, Au, Pd, Pt and Cu can be galvanically deposited onto Si powder out of solutions of HF(aq). Nucleation of deposited metal nanoparticles is made more uniform by dispersing the Si powder in acetic acid and injecting the dissolved metal ions at a controlled rate. The use of a syringe pump to deliver not only metal ions during deposition but also the oxidant (H2O2) during MACE is essential for increased product uniformity and yield. Temperatures of ~0–50 °C and grades of Si (i.e. electronics grade Si with various doping levels or metallurgical grade Si) produced significantly different pore size distributions. Doping levels and the associated changes in band bending and space charge layer width4 are linked to changes in the pore size distribution.As shown in Fig. 1(A), high loading of Ag resulted in deposition of dendrites and Ag particles. These broke into ~50–100 nm Ag nanoparticles that etched in a correlated fashion to generate parallel etch track pores. The uniform size distribution of the Ag nanoparticles was generated dynamically during etching. This conventional high-load MACE (HL-MACE) did not produce Si nanowires (Si NWs) directly. However, etch track pore walls were cleaved readily by ultrasonic agitation to form Si NWs. Etching far removed from the metal nanoparticles (remote etching) also occurred for highly doped Si.As shown in Fig. 1(B), reducing the loading of Ag by a factor of 100 to 1000 compared to HL-MACE created 10–30 nm nanoparticles that etched in an uncorrelated fashion and generated randomly directed etch track pores. Significant etching far removed from the Ag catalyst also occurred. The combination of localized and remote etching resulted in a bimodal distribution of mesoporosity peaking at ~4 nm from remote etching and 10–30 nm from localized etching. The bimodal distribution was confirmed by the BJH method of nitrogen adsorption porosimetry. The pore size distribution could be controlled by changing the process temperature, the grade of Si and the metal composition. Whereas Ag, Au, Pd and Pt performed well in both the HL and LL limits, Cu generated porosification only in the LL limit.Funded by grants #314552 and #314412 from Academy of Finland and the National Science Foundation award #1825331. The microscopy studies were performed using the facilities in the UConn/Thermo Fisher Scientific Center for Advanced Microscopy and Materials Analysis (CAMMA). 1 K. Tamarov, J. D. Swanson, B. A. Unger, K. W. Kolasinski, A. T. Ernst, M. Aindow, V.-P. Lehto, and J. Riikonen, ACS Appl. Mater. Interfaces 12, 4787 (2020). 2 K. W. Kolasinski, W. B. Barclay, Y. Sun, and M. Aindow, Electrochim. Acta 158, 219 (2015). 3 K. W. Kolasinski, N. J. Gimbar, H. Yu, M. Aindow, E. Mäkilä, and J. Salonen, Angew. Chem., Int. Ed. Engl. 56, 624 (2017). 4 E. Torralba, S. Le Gall, R. Lachaume, V. Magnin, J. Harari, M. Halbwax, J.-P. Vilcot, C. Cachet-Vivier, and S. Bastide, ACS Appl. Mater. Interfaces 8, 31375 (2016). Figure 1

Luminous power improvement in InGaN V-Shaped Quantum Well LED using CSG on SiC Substrate

This paper presents the design and simulation of Silicon Carbide (SiC) based technology, Indium Gallium Nitride (InGaN) Multiple Quantum Well (MQW) Light-Emitting Diode (LED) with a Compositionally Step Graded (CSG) InGaN barrier and V-Shaped well in the active region. The simulations are obtained in Silvaco Computer Aided Design simulator and parameters such as Internal Quantum Efficiency (IQE) with respect to input current, spontaneous emission in regard to wavelength and power versus current in the device are theoretically studied. The CSG InGaN barrier LED with V-shaped quantum well shows substantial growth in output power when compared to the CSG GaN barrier structure with conventional MQW. The high carrier confinement in the V-shape well causes, transportation/injection of hole and change in band bending due to polarization effect. Moreover, lattice-matched SiC substrate over GaN material increases the InGaN V-shaped MQW LEDs radiative recombination rate which in turn leads to high output power. The optical luminous power of 160mW and 82% of peak IQE, emitting wavelength at 460 nm and 200mA of injection current is obtained for the proposed LED. The enactment of the V shape MQW CSG-InGaN device technology is a good alternative choice for commercial and industrial lighting applications.

Band bending of TiO2 induced by O-xylene and acetaldehyde adsorption and its effect on the generation of active radicals

Most studies on the photodegradation of volatile organic compounds (VOCs) have focused on the synthesis of efficient photocatalysts. However, little attention has been paid to the band bending change of semiconductive photocatalysts after the adsorption of VOCs. Herein, we first disclose how the adsorption of two typical VOCs influences the band bending of P-type rutile TiO2 and consequently changes the amount of reactive radicals. This provides a new way to understand the experimental phenomenon of heterogeneous reactions. Theoretical computations of the adsorption model and zeta potential tests both verified that o-xylene is an acceptor molecule when it adsorbs on the TiO2 surface, and it tends to attract electrons from TiO2. In contrast, acetaldehyde is a donor molecule. A distinct electron transfer direction between TiO2 and adsorbed molecules (o-xylene and acetaldehyde) induces a different band bending degree. O-xylene adsorption alleviates the downward band bending of TiO2 itself, whereas acetaldehyde adsorption strengthens the downward band bending. The probability of electrons and holes reaching the TiO2 surface is influenced by this change, which has a considerable influence on the generation of active radicals. Consequently, o-xylene adsorption leads to more hydroxyl radical generation, and acetaldehyde adsorption results in less hydroxyl radical generation. As a result, hydroxyl radicals play the predominant role in the degradation of o-xylene, whereas the photocatalysis of acetaldehyde is dominant for superoxide radicals. In addition, the band bending of a semiconductor induced by gaseous molecule adsorption has the potential for application in gas sensors to improve sensitivity.

Directional Change of Interfacial Electric Field by Carbon Insertion in Heterojunction System TiO2/WO3.

Z-scheme transfer is an ideal photocatalytic system with stronger redox ability, but its design and construction still lack understanding. Herein, the work function difference and the band bending are found to be the determining factors for the construction of the Z-scheme transfer mechanism of photoexcited charges in TiO2/WO3. The control of work function and band bending achieved by carbon insertion results from the hybridization of orbitals and redistribution of electron density, as demonstrated by ultraviolet photoelectron spectroscopy and photocatalytic analysis. The heterojunction system, TiO2/WO3, with controlled work function and band bending, shows 2 times faster •OH radical formation rate (0.011 μmol min-1) compared to the undisturbed system. First-principles calculation reveals that the changes in work function and band bending result in an interfacial electric field, which shifts the charge transfer mechanism from type II to Z-scheme. This work proves that the design of work function and band bending allows reconstructing charge transfer mechanism by forming the interfacial electric field in heterojunction systems.

Charge Separation, Band-Bending, and Recombination in WO3 Photoanodes.

In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide/electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all time scales from picoseconds to seconds and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending, and oxygen vacancy defect state occupancy.

Gap states induce soft Fermi level pinning upon charge transfer at ZnO/molecular acceptor interfaces

The deposition of strong molecular electron acceptors onto ZnO induces a substantial work function (\ensuremath{\phi}) increase due to electron transfer from the inorganic semiconductor to the molecules. The \ensuremath{\phi} increase results from two mechanisms: (i) a change of the surface band bending within ZnO and (ii) an interface dipole between the inorganic surface and the negatively charged acceptors. The molecule adsorption induced upward band bending in ZnO is, however, found to be limited to a few 100 meV, while the \ensuremath{\phi} increase is significantly larger (up to 2.8 eV). We elucidate the origin of limited upward band bending by revealing a notable gap state density-of-states (GDOS) using high-sensitivity photoemission spectroscopy. Upon acceptor-induced upward band bending, the GDOS with a wide energy distribution becomes increasingly unoccupied. This, in turn, makes the interface dipole dominant and limits the ZnO surface band bending changes due to a ``soft'' pinning at the Fermi level.

Size-dependent surface potential of Si-doped InN nanorods and the role of inhomogeneous free-electron distribution

We carry out the surface potential (SP) measurements of Si-doped InN nanorods (NRs) grown by plasma-assisted molecular beam epitaxy. Photoluminescence and photoemission spectroscopic studies reveal that the Si-doped InN nanorods possess surface electron accumulation. To estimate the SP value of the InN nanorods, a contact potential difference is measured using Kelvin probe force microscopy (KPFM). In order to avoid the influence of the surface adsorbed species, KPFM measurements were carried out at a high vacuum condition of ∼7.5 × 10−7 mbar. The SP value of the Si-doped InN nanorods is found to depend on the size of nanorods. The size-dependent SP value of the Si-doped InN nanorods is attributed to the variation in the downward surface band bending caused by the change in the sheet carrier density of surface electron accumulation. The change in surface band bending is the result of the variation in the free-electron distribution with a size of the NRs.

Impact of surface states and bulk doping level on hybrid inorganic/organic semiconductor interface energy levels

In applications, surface states and bulk doping concentration are important parameters of inorganic semiconductors, as they determine the bulk properties and substantially influence the properties of interfaces in devices, foremost the electron energy level alignment. In this work, we provide a qualitative model to describe the influence of surface state density and bulk donor concentration on the work function increase upon deposition of strong organic molecular acceptors onto the surface of n-doped inorganic semiconductors. This work function increase due to electron transfer to the molecular layer has two contributions: the formation of an interface dipole and a change of the near-surface space charge region inside the inorganic semiconductor, referred to as surface band bending. By using different surface preparation methods, we show how the surface state density limits the surface band bending change and enhances the interface dipole, both measured independently by photoelectron spectroscopy. In addition, we show that bulk donor concentration variation of the inorganic semiconductor has minor influence on the ratio of the two contributions to the work function change, at least for low to moderate donor concentrations up to 1019 cm−3.

Sarin-simulant detection by phthalocyanine/palladium structures: From modeling to real sensor response

The full recognition of a sensing mechanism is a key issue in the gas sensors investigation. Therefore within presented work we proposed the new approach for the comprehensive sensing mechanism description: theoretical analysis and experimental investigation of the sensing structure. The object of our study was metal-free phthalocyanine/palladium structure utilized in sarin-simulant dimethyl methylphosphonate (DMMP) detection at room temperature. In the theoretical evaluation semi-empirical simulations have been performed revealing bonding energies between sensing materials and DMMP molecule. Different variants of metallic overlayer were considered in the modeling including bare palladium and palladium oxide (PdO). The results of the simulations indicated physisorption as a main sensing mechanism, especially enhanced in presence of PdO phase. Theoretical estimation of electron affinity and band bending changes during gas adsorption was conducted indicating strong impact of surface dipole effect. The experimental investigations was based on X-ray photoelectron spectroscopy (XPS) chemical characterization of DMMP exposed surface supported by atomic force microscopy imaging. The XPS results coincided with theoretical statements demonstrating no traces of chemical interaction with analyzed gas. Simultaneously, the presence of PdO was corroborated. Finally, the real sensor response was analyzed confirming outcome of the theoretical simulations and results of the experimental investigations.

Surface State Density Determines the Energy Level Alignment at Hybrid Perovskite/Electron Acceptors Interfaces

Substantial variations in the electronic structure and thus possibly conflicting energetics at interfaces between hybrid perovskites and charge transport layers in solar cells have been reported by the research community. In an attempt to unravel the origin of these variations and enable reliable device design, we demonstrate that donor-like surface states stemming from reduced lead (Pb0) directly impact the energy level alignment at perovskite (CH3NH3PbI3-xClx) and molecular electron acceptor layer interfaces using photoelectron spectroscopy. When forming the interfaces, it is found that electron transfer from surface states to acceptor molecules occurs, leading to a strong decrease in the density of ionized surface states. As a consequence, for perovskite samples with low surface state density, the initial band bending at the pristine perovskite surface can be flattened upon interface formation. In contrast, for perovskites with a high surface state density, the Fermi level is strongly pinned at the conduction band edge, and only minor changes in surface band bending are observed upon acceptor deposition. Consequently, depending on the initial perovskite surface state density, very different interface energy level alignment situations (variations over 0.5 eV) are demonstrated and rationalized. Our findings help explain the rather dissimilar reported energy levels at interfaces with perovskites, refining our understanding of the operating principles in devices comprising this material.

Effect of ammonium-sulfide solvent on the surface passivation of GaSb (100)

X-ray photoemission spectroscopy and scanning electron microscopy are used to study the effect of a solvent on the chemical composition of a sulfide passivating layer formed on GaSb (100) surface. Treatment of the GaSb (100) surface with ammonium-sulfide solutions results in the formation of a thick (several tens of nanometers) coating composed of a mixture of various binary and ternary gallium and antimony sulfides. The chemical compositions of the passivating sulfide layers formed in aqueous and alcoholic ammonium-sulfide solutions are different. Annealing of the sulfide layer at 360°C in ultrahigh vacuum leads to the partial desorption of antimony from the surface, the decomposition of ternary sulfides into their binary constituents, and change in the surface band bending of this layer.