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The influence of electron-withdrawing substituents on the photoelectrochemical properties of phthalocyanines is shown in a comparison between hexadecafluorophthalocyaninatozinc(II) (F₁₆PcZn) and the unsubstituted phthalocyaninatozinc(II) (PcZn). The role of surface states in the photoelectrochemistry of both materials has been investigated by time-resolved photocurrent measurements in the millisecond range. The charging and discharging of surface states could clearly be seen as spikes at the beginning and the end of illumination. Surface states were filled with photogenerated electrons at PcZn and with photogenerated holes at F₁₆PcZn. In the steady state under illumination only cathodic photocurrents were detected at PcZn, while at F₁₆PcZn both cathodic and anodic photocurrents were observed. An adsorption step of electroactive species prior to the charge transfer was derived from the dependence of the steady state photocurrents on the electrolyte concentration for both materials. The concentration dependence of the charging and discharging currents, however, showed that charge transfer from surface states to the electrolyte occurs at PcZn, while at F₁₆PcZn the surface states only represent recombination centres.

Two-dimensional simulations of hot electron and quantum effects in AlGaN/GaN HEMT devices are performed. The effect of surface states is accounted for by defining effective net surface charges in our simulation. Simulation results show that surface states and surface traps which model the electron depletion play an important role in the electrical characteristic of AlGaN/GaN HEMT. Hot electrons play an important role in the charge transport by spilling over into the bulk GaN where they are captured by traps. Excessive net surface charges make the 2DEG much hotter through the role of vertical electric field above the channel. Lattice scattering and electron tunneling are enhanced with the increasing density of net surface charges, and electron mobility and saturation velocity are significantly reduced.

The electronic properties of the bulk CsPbI₂Br and the passivation of CsPbI₂Br (110) surface states are calculated by using first-principles calculations. It is found that the band gap of CsPbI₂Br is 1.42eV by using the generalized gradient approximation of the Perdew–Burke–Ernzerhof function. The band gap is about 1.96eV by using the more complex Heyd–Scuseria–Ernzerhof mixed functional, which is closer to the experimental value of 1.92eV. The valence band top of CsPbI₂Br bulk is mainly contributed by I-5p orbital and Br-4p orbital, and the conduction band bottom is mainly contributed by Pb-6p orbital. Through the calculation of CsPbI₂Br (110) surface states passivated by Cl, F and H atoms, it is found that H atom has the best passivation effect. Its adsorption energy value fluctuates less, it is less sensitive to the adsorption position, and the adsorption is stable. Followed by F atom, its passivation effect is worse than that of H atom, but better than that of Cl atom. Although the passivation position has a certain influence on it, it has little effect. The Cl atom is most affected by the passivation position. The different positions of passivated atoms have a significant impact on adsorption energy. The adsorption stability is poor, and the passivation effect is also poor. Through analyzing the charge density difference and Bader charges, it is found that the H atom gets more electrons from the I atom, which is beneficial to passivate surface states. The electron-acquiring ability of the F atom is inferior to that of the H atom and is superior to that of the Cl atom. The electron-acquiring ability and passivation ability of Cl atom are the weakest among the three elements.

This paper deals with the correlation of surface states and photoactivity of a Pt/TiO₂ catalyst prepared by means of photoreduction of on anatase nano-TiO₂ powders. Conductivity measurements of pressed powders and XPS were applied in analysis of the surface states energy levels and chemical states of Pt at the nano-TiO₂ surface, respectively. The results revealed a new surface state with the energy level of 0.35 eV, lower than the conduction band edge of TiO₂. According to the XPS analysis, the new surface state can be attributed to the Pt⁰ clusters deposited on the surface of TiO₂ particles. The photoactivity of the as-prepared Pt/TiO₂ catalyst was characterized by photodecoloration of brilliant red X-3B in aqueous solution. The experimental results indicated that the photoactivity of the Pt/TiO₂ catalyst is much higher than that of TiO₂. The surface Pt⁰ clusters possibly function as trapping centers of generated electrons, resulting in the promotion of photocatalytic activity.

We analyze the electronic and magnetic characteristics of the Co₂FeGa Heusler compound and the surface (001) Co₂FeGa with the $L{2}_1$ structure based on first principles calculations. The Co₂FeGa Heusler possesses high Curie temperatures and magnetization, and is therefore the most studied composition for spintronics applications. We examine the influences of (001) surface and relate interactions on the half-metallicity and the magnetic properties with two ideal terminations named Co–Co and Fe–Ga and modified Co– terminated (001) surfaces. We notice that the half-metallicity seen in the bulk Co₂FeGa is destroyed at all terminations, the spin polarization is smaller than that the bulk, the values of the Co–Co, Fe–Ga and Co– terminations are 50%, 27% and 43% respectively, but all surfaces are shown to be metallic due to the surface states staying at the Fermi level despite the confirmed $t_{2g}-e_g$ band splitting caused by the liaison influence. The calculated magnetic moment of these terminations was found for all the subsurface to be close to that of the bulk system and this makes this compound of maximum benefit in the pilot applications of spintronic systems.

Semiconductor gas sensors based on nanocrystalline SnO₂ offer many advantages over current technologies for detecting reducing gases, such as low cost, long lifetime, and high selectivity and sensitivity. However, the local surface properties on the nanoscale of SnO₂ nanocrystals are not fully understood, which impedes the exploitation of the full potential of SnO₂ for gas sensing applications. In this paper, we present a scanning tunneling microscopy and spectroscopy (STM/STS) study of nanocrystalline SnO₂ at room temperature, and under standard sensing conditions at 120°C. STS data indicate that the electronic surface properties change with nanoparticle size, temperature and exposure to gas. The surface density of states in the band gap is shown to increase with temperature while CO exposures induce a large drop in the density of band gap states as the CO molecules react with chemisorbed oxygen species.

Scanning tunneling microscopy (STM) and spectroscopy have been performed on a nanostructure "line-like pattern" appearing on atomically flat terraces of bcc-Fe(001) with a c(2 × 2) reconstructed structure. Atomically resolved STM reveals that the line-like pattern consists of regularly aligned surface atoms that have a similar symmetry to the (1 × 1) structure of Fe(001). The line-like pattern plays a role of domain boundaries of c(2 × 2) domains. The patterns exhibit bias-voltage dependent image contrast in the STM topographic images. In negative sample bias-voltage range the patterns appear lower than the c(2 × 2) domains and higher in positive range. In dI/dV tunneling spectra taken on the patterns a shoulder is observed around +0.2 V. On the other hands, a small salient is distinguished around +0.4 V in the spectra taken on the c(2 × 2) domains. These features are originated from surface states existing on bcc-Fe(001) surfaces with (1 × 1) structure and c(2 × 2) reconstructed structure, respectively. Emergence of the line-like patterns results in the difference of tunneling conductance.

We report here the UV-VIS study of CdS nanoparticles synthesized via Chemical precipitation in aqueous medium, the UV-VIS of CdS in aqueous medium shows an excitonic peak at 230 nm, while that of CdS quantum dots in solid form shows an absorption maximum at 480 nm. This bleaching of band gap may be attributed to Moss–Burstein shift in the absorbance edge.

Tunneling transmission of electrons excited by the sun light in the bulk TiO₂ to its surface states is simulated. The model is built for the TiO₂/Si nanostructure. The tunneling transmission coefficient was calculated by the phase functions method. The surface states are considered to be formed due to adsorption of impurities and organic compounds on TiO₂. Energy dependence of the transmission coefficient is influenced by an interference of transmitted and reflected waves under conditions of the complex potential relief.

Carbon dots (CDs), as an important carbon-based nanomaterial, have been widely used in fluorescence (FL) detection, bioimaging, photocatalysis, light-emitting devices (LEDs) and many other fields, owing to their excellent optical properties. Regulating the fluorescent properties of carbon dots (CDs) in a controlled manner has been a fundamental research goal. Herein, solvent-dependent CDs (N-CDs) are synthesized through the solvothermal reaction of $o$-phenylenediamine (OPD) in ethanol. When N-CDs are dispersed in solvents of different polarities, the emission wavelength only turned 39nm red-shift. While, the O-CDs are obtained by modifying the surface of N-CDs with 1,2,4-benzenetricarboxylic acid (TMA). O-CDs show four obvious FL emission peaks in the yellow, orange and red regions at 554nm, 604nm, 650nm and 712nm with a tail extending to 750nm, even entering into near-infrared-I (NIR-I) region. Thus, according to the special structure of CDs, this work provides a novel donor-$\pi$-acceptor (D-$\pi$-A) strategy for preparing long wavelength emissive CDs. Finally, the N-CDs and O-CDs are successfully applied to cellular imaging.

PbTiO₃ nanorods with tetragonal phase were synthesized by hydrothermal method and heat treatment, and temperature-dependent electrical transport properties of individual PbTiO₃ nanorod were investigated. The results show that the conductivities of PbTiO₃ nanorods are gradually enhanced with temperature increasing from 77.4 to 295K, and exhibit the typical nonlinear I–V characteristics. The barrier height between Au electrode and nanorod is reduced from 0.137 to 0.088eV with increasing bias from 0.2 to 1V. The corresponding values of thermal activation energies are 0.172 and 0.06eV below the conduction band for 180–295 and 77.4–180K, respectively. This semiconductor-like behavior may result from the larger number of surface defects or localized states in the amorphorized PbTiO₃.

A quantum device is an electronic component whose electrical properties can only be understood from quantum mechanical phenomena, such as the wave nature of electrons or the discreet energy levels of a confined electron. A couple of criteria must be met before quantum observations can be made. First, the thermal energy must be less than the electron energy-level spacing which typically requires a combination of temperatures less than 1 K (−272°C) and device dimensions less than one micron (micron = 1 μm = 10⁻⁶ m). Second, electron motion must be ballistic, meaning no scattering takes place within the confines of the device, which requires small devices and material of extremely high purity. Quantum devices have been studied intensely for the past 20 years, partly because they provide an ideal arena to investigate diverse quantum-mechanical phenomena, but also to identify and study the next generation of electronic components whose mass production will soon be possible if microchip miniaturization continues at the current rate. The simplest quantum device is a two-dimensional electron gas (2DEG), an electron sheet, where electrons are confined in one dimension and free in two. Confinement by electric fields occurs at the junction between layers of the compound semiconductors GaAs and AlGaAs. Confinement in further dimensions, to create quantum wires and dots for example, is usually achieved using patterned surface electrodes fabricated by electron beam lithography (EBL). During experiments a negative bias is applied to the surface electrodes which expel electrons from the 2DEG lying underneath the electrodes. The quantum wires or dots are created in the 2DEG plane under gaps between the electrodes. Erasable electrostatic lithography (EEL) is a flexible alternative to EBL. Using a charged scanning probe, patterns of negative charge are drawn on a GaAs sample surface. Electrons are expelled from the 2DEG underneath the surface charge, and the quantum device is created in the 2DEG plane under gaps in the surface charge. EEL has the unique advantages that fabrication takes place in the same low-temperature high-vacuum environment required for measurement and that patterning is erasable. The charge, and therefore the quantum device, can be erased either locally using the scanning probe or globally by illuminating the device with a flash of red light. This means that as part of the experiment, the device geometry can be modified without constraint and the device tuned for optimum performance. With EEL many new fascinating experiments become possible and often with reduced fabrication timescales.

Clean metal surfaces often display an atomic arrangement at the surface that differs from the one in the bulk. Some of these surface reconstructions show mesoscopic order and are very adequate to act as a template for the ordered growth of arrays of atoms, molecules or clusters. The electronic states at some surfaces can be prototypes of highly dense 2D electron gases where a number of fundamental properties can be addressed in detail. Localized surface states, on the other hand, are relevant in chemical processes at surfaces. The recent developments in experimental and theoretical techniques allow the exploration of these issues with unprecedented precision.

Topological insulators are new quantum states with helical gapless edge or surface states inside the bulk band gap. These topological surface states are robust against weak timereversal invariant perturbations without closing the bulk band gap, such as lattice distortions and non-magnetic impurities. Recently a variety of topological insulators have been predicted by theories, and observed by experiments. First-principles calculations have been widely used to predict topological insulators with great success. In this review, we summarize the current progress in this field from the perspective of first-principles calculations. First of all, the basic concepts of topological insulators and the frequently-used techniques within first-principles calculations are briefly introduced. Secondly, we summarize general methodologies to search for new topological insulators. In the last part, based on the band inversion picture first introduced in the context of HgTe, we classify topological insulators into three types with s–p, p–p and d–f, and discuss some representative examples for each type.