Surface Photovoltage Imaging Research Articles

Enhanced photocatalytic hydrogen production and simultaneous benzyl alcohol oxidation by modulating the Schottky barrier with nano high-entropy alloys

Catalysts play a vital role in photocatalytic water-splitting by converting solar energy into storable chemical energy. In this study, we successfully synthesized an HCN/HEA heterostructure by rationally combining the Pt18Ni26Fe15Co14Cu27 nano-high-entropy alloy (HEA) as an effective cocatalyst with protonated g-C3N4 (HCN) nanosheets, via an electrostatic self-assembly method. The resulting HCN/HEA exhibited remarkable performance in both photocatalytic H2 production and selective oxidation of benzyl alcohol to benzaldehyde. The integration of HEA into HCN leads to the formation of Schottky junctions, which can significantly accelerate charge migration and reduce the recombination of charge carriers. Further investigations using in situ surface photovoltage imaging demonstrated that the reducing cocatalyst HEA can act as a photogenerated electron trap. The best HCN/HEA heterojunction exhibited excellent photocatalytic hydrogen production activity, reaching 2.4 mmol g−1 h−1 and an impressive benzaldehyde production rate of 5.44 mmol g−1 h−1. These values were 958 and 6.6 times higher than those achieved with pristine HCN, respectively. This study offers promise for the rational design of high-performance cocatalysts to improve the transport, separation, and utilization of light-release charge carriers.

Enhancement in the photocatalytic H2 production activity of CdS NRs by Ag2S and NiS dual cocatalysts

Cocatalysts play an indispensable role in photocatalytic H2 production via water splitting for the conversion of solar energy into storable chemical energy. Herein, the hybrid catalyst CdS/Ag2S/NiS is synthesized via hydrothermal and photodeposition methods. CdS/Ag2S/NiS shows a drastically elevated hydrogen production rate of 48.3 mmol g−1 h−1 under visible light due to the combined merits of the Schottky junction between CdS and metal-like Ag2S and the constructed p–n junction between CdS and NiS. Time-resolved photoluminescence spectroscopy and photochemical tests reveal the accelerated charge transfer and significantly reduced electron–hole pair recombination. Further investigation with in-situ surface photovoltage imaging technology demonstrates that the reduction cocatalyst Ag2S and oxidation cocatalyst NiS can serve as photogenerated electron and hole traps, respectively. This research not only provides insight into designing high-efficiency photocatalyst for hydrogen production but also utilize a brand new method for the confirmation of charge–carrier migration pathways.

A scanning probe microscopy study of nanostructured TiO2/poly(3-hexylthiophene) hybrid heterojunctions for photovoltaic applications.

The nanoscale morphology of photoactive hybrid heterojunctions plays a key role in the performances of hybrid solar cells. In this work, the heterojunctions consist of a nanocolumnar TiO2 surface covalently grafted with a monolayer of poly(3-hexylthiophene) (P3HT) functionalized with carboxylic groups (–COOH). Through a joint analysis of the photovoltaic properties at the nanoscale by photoconductive-AFM (PC-AFM) and surface photovoltage imaging, we investigated the physical mechanisms taking place locally during the photovoltaic process and the correlation to the nanoscale morphology. A down-shift of the vacuum level of the TiO2 surface upon grafting was measured by Kelvin probe force microscopy (KPFM), evidencing the formation of a dipole at the TiO2/P3HT-COOH interface. Upon in situ illumination, a positive photovoltage was observed as a result of the accumulation of photogenerated holes in the P3HT layer. A positive photocurrent was recorded in PC-AFM measurements, whose spatial mapping was interpreted consistently with the corresponding KPFM analysis, offering a correlated analysis of interest from both a theoretical and material design perspective.

Positioning the Water Oxidation Reaction Sites in Plasmonic Photocatalysts.

Plasmonic photocatalysis, stemming from the effective light absorbance and confinement of surface plasmons, provides a pathway to enhance solar energy conversion. Although the plasmonic hot electrons in water reduction have been extensively studied, exactly how the plasmonic hot holes participate in the water splitting reaction has not yet been well understood. In particular, where the plasmonic hot holes participate in water oxidation is still illusive. Herein, taking Au/TiO2 as a plasmonic photocatalyst prototype, we investigated the plasmonic hot holes involved in water oxidation. The reaction sites are positioned by photodeposition together with element mapping by electron microscopy, while the distribution of holes is probed by surface photovoltage imaging with Kelvin probe force microscopy. We demonstrated that the plasmonic holes are mainly concentrated near the gold-semiconductor interface, which is further identified as the reaction site for plasmonic water oxidation. Density functional theory also corroborates these findings by revealing the promotion role of interfacial structure (Ti-O-Au) for oxygen evolution. Furthermore, the interfacial effect on plasmonic water oxidation is validated by other Au-semiconductor photocatalytic systems (Au/SrTiO3, Au/BaTiO3, etc.).

High-resolution noncontact AFM and Kelvin probe force microscopy investigations of self-assembled photovoltaic donor-acceptor dyads.

Self-assembled donor–acceptor dyads are used as model nanostructured heterojunctions for local investigations by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). With the aim to probe the photo-induced charge carrier generation, thin films deposited on transparent indium tin oxide substrates are investigated in dark conditions and upon illumination. The topographic and contact potential difference (CPD) images taken under dark conditions are analysed in view of the results of complementary transmission electron microscopy (TEM) experiments. After in situ annealing, it is shown that the dyads with longer donor blocks essentially lead to standing acceptor–donor lamellae, where the acceptor and donor groups are π-stacked in an edge-on configuration. The existence of strong CPD and surface photo-voltage (SPV) contrasts shows that structural variations occur within the bulk of the edge-on stacks. SPV images with a very high lateral resolution are achieved, which allows for the resolution of local photo-charging contrasts at the scale of single edge-on lamella. This work paves the way for local investigations of the optoelectronic properties of donor–acceptor supramolecular architectures down to the elementary building block level.

Electronic Defect and Contamination Monitoring in Si Wafers Using Spectrally Integrated Photocarrier Radiometry

The ability of spectrally integrated room-temperature photocarrier radiometry to image electronic defects and contamination in silicon wafers is presented. Amplitude and phase imaging contrast is a result of signal sensitivity to local variations in the recombination lifetime of photoexcited carriers. Experimental frequency scans are fitted to carrier density-wave theory to simultaneously obtain the recombination lifetime, diffusivity, and surface recombination velocities (front and back). Lifetime measurements are combined with lateral surface scans to produce quantitative lifetime imaging. Contaminated boron-doped silicon wafers with iron concentration on a baseline of have been successfully imaged and exhibit a much higher spatial resolution than surface photovoltage images. Recombination lifetime measurements before and after photodissociation of FeB pairs have been utilized for quantitative determination of iron concentration; however, lifetime accuracy limitations due to photodissociation by the excitation laser under medium-to-high optical injection levels required for photocarrier radiometric measurements compromise the accuracy of heavy ion (Fe) concentration measurements.

The effect of adsorbed oxygen on the surface potential of n-GaAs(110)

Potential variations on semiconductor surfaces are often mapped using a combination of constant current topographic and local surface photo-voltage (SPV) imaging. SPV imaging provides a direct measurement of surface-potential variations at large lateral distances from a charged defect or adsorbate. However, directly above the defect, variations in the SPV signal need to be interpreted in terms of surface screening, traps, and band bending. We have examined these effects using isolated oxygen atoms on a GaAS(110) surface, which is free of surface states. We interpret variations in the SPV signal in terms of a simple electrostatic model which considers the oxygen-induced Coulomb potential and corresponding image potential, both of which affect the surface density of states.

Scanning tunneling microscopy of single dye molecules on GaAs(1 1 0) surfaces

Scanning tunneling microscopy and surface photovoltage images are reported for isolated dye molecules on the GaAs(1 1 0) surface. Profound differences in the molecular images are observed for different experimental conditions. Specifically, contrast variations with changing bias polarity and magnitude, and for different substrate doping type are examined. Several mechanisms are considered to describe the observations including direct and resonant tunneling, and switching of charges between molecular states in the band gap and the energy bands of the GaAs. This treatment enables both assignment of the molecular charge state polarity, and clarification of the mechanism for current flow.

Quasiparticle Calculations for Point Defects on Semiconductor Surfaces

We discuss the implementation of quasiparticle calculations for point defects on semiconductor surfaces and, as a specific example, present an ab initio study of the electronic structure of the As vacancy in the +1 charge state on the GaAs(110) surface. The structural properties are calculated with the plane-wave pseudopotential method, and the quasiparticle energies are obtained from Hedin's GW approximation. Our calculations show that the 1a" vacancy state in the band gap is shifted from 0.06 to 0.65 eV above the valence-band maximum after the self-energy correction to the Kohn-Sham eigenvalues. The GW result is in close agreement with a recent surface photovoltage imaging measurement.

The electronic nature of the Cl adatom at GaAs(1 1 0) surfaces due to Cl 2 dissociative adsorption and CCl 4 photodecomposition

Clean, well defined GaAs(1 1 0) surfaces at room temperature, were exposed to carbon tetrachloride vapor. Only when illuminated during the exposure (with 400 mW/cm 2 of 532 nm light) could adsorbed species be observed with scanning tunneling microscopy (STM) on flat terraces. The features in the STM images were compared to those due to exposure to Cl 2 gas in the dark, and the CCl 4 decomposition products were shown to be identical to chlorine adatoms which are bound to surface Ga atoms (A-type). Since the Cl adatoms appear dark at low positive sample bias, and bright at high bias, and from the absence of localized contrast in atomically-resolved surface photovoltage images, we deduce that the Cl adatom interacts with the Ga-induced C 3 surface resonance, that it (or the Cl–Ga complex) is not charged, and it does not give rise to any surface state in the GaAs band gap that could pin the Fermi level.

Surface photovoltage imaging for the study of local electronic structure at semiconductor surfaces

Atomically-resolved surface photovoltage (SPV) imaging with a scanning tunnelling microscope was used by several laboratories to reveal the local degree of sub-surface electronic band bending at and around defects and adsorbates on various semiconductor surfaces. It turns out that the scanning tunnelling microscope tip-induced electric field, and/or the charging of surface states by the tunnelling current, can affect the band bending under the tip and help us learn more about the surface local electronic structure. This review features an explanation of the band bending phenomenon and how it gives rise to SPV, SPV imaging methods, a coverage of the experimental work done up to this date, and finally, a discussion about the source of atomically resolved features in the SPV maps.

Local Fermi-level pinning at a single adatom (Cs) or vacancy (As) on a GaAs(110) surface

Local surface photovoltage imaging with a scanning tunneling microscope is used to demonstrate two types of local Fermi-level pinning next to a single adsorbate or defect, taking advantage of the tip-induced band bending due to the lack of gap states at clean GaAs(110) surfaces. The pinning energy at a single cesium adatom location was determined to be $0.56\ifmmode\pm\else\textpm\fi{}0.04\mathrm{eV}$ above the valence-band maximum (VBM), while for As vacancy, the Fermi level cannot go below the top of a filled surface state, $0.62\ifmmode\pm\else\textpm\fi{}0.04\mathrm{eV}$ above the VBM.

Single Cs adatoms at Si(001)2×1 surfaces: a local surface photovoltage imaging study

Local surface photovoltage (SPV) and current imaging, using a scanning tunneling microscope (STM), were utilized in order to elucidate the effect of a single adsorbed cesium atom on the local electronic structure of p- and n-type Si(001)2×1 surfaces, in its vicinity. The Cs adatom gives rise to a small depression at negative sample bias constant current topography (CCT) image. The opposite sides of two adjacent Si dimers appear as protrusions. SPV and current imaging show that the Fermi level is ‘softly’ pinned at the surface in the location of these protrusions, possibly because of a downshift and/or broadening of the π ∗ band and its screening of the Cs positive charge.

The role of adsorbed alkali metal atoms in the enhancement of surface reactivity: A scanning tunneling microscopy study of low coverage K/Si(111)7×7 surfaces

Atomically resolved surface photovoltage imaging and tunneling barrier imaging were performed with a scanning tunneling microscope to elucidate the nature of the initial adsorption state of potassium on room temperature Si(111)7×7 surface, which appears as a missing center Si-adatom. No evidence was found to support the fact that this adsorption mode is charged. K islands start to grow (and accumulate at 7×7 domain boundaries) already at coverages as low as 4×1012 cm−2. These islands may be responsible for the previously reported work function lowering and enhanced chemical reactivity upon K deposition.

Electrostatic sample-tip interactions in the scanning tunneling microscope.

Local surface photovoltage (SPV) measurements were used to measure how the electric field of a scanning tunneling microscope tip perturbs the electronic band structure at Si(001), Si(111)-(7\ifmmode\times\else\texttimes\fi{}7), and H-terminated Si(111) surfaces. The results demonstrate that tip-induced band bending is important under typical STM conditions even on surfaces whose surface Fermi levels are nominally ``pinned.'' Spatially resolved measurements of band bending as a function of sample bias show that atomic-scale contrast in SPV images can result from local variations in the ability of the surface states under the tip to screen external electric fields.

Observation of Ring-Distributed Microdefects in Czochralski-Grown Silicon Wafers with a Scanning Photon Microscope and Its Diagnostic Application to Device Processing

A scanning photon microscope (SPM) based on ac surface photovoltage imaging is applied to observe oxygen-related microdefects which are distributed in a ring in oxidized Czochralski-grown silicon wafers, and morphological and microstructural characteristics of the microdefects are then analyzed. The overall distribution of the ring-shaped region revealed by the SPM correspond well to that observed with X-ray topography. The SPM is able to differentiate deteriorated regions as different image contrasts, where stacking faults or oxide precipitates accompanying punched-out dislocation loops exist.

Nondestructive Observations of Surface Flaws and Contaminations in Silicon Wafers by Means of a Scanning Photon Microscope

A scanning photon microscope based on ac surface photovoltage imaging was applied for nondestructive observations of surface scratches and surface contaminations by SO42- in 125 mm diameter n-type silicon wafers. The scratches and contaminations create bulky and interface traps, resulting in lower ac surface photovoltages. The ac surface photovoltages attained between 35 through 150 µV have a relationship with the concentration of SO42- analyzed by ion chromatography ranging between 102 and 2×104 ng/wafer, implying that the present method is useful and simple for evaluation of the contaminated level of wafer surfaces in a nondestructive and noncontact manner.

I-6 scanned surface photovoltage images of defects on semiconductors

I-6 scanned surface photovoltage images of defects on semiconductors