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Nanoscale Characterization
Published in Ram K. Gupta, Sanjay R. Mishra, Tuan Anh Nguyen, Fundamentals of Low Dimensional Magnets, 2023
Arvind Kumar, Swati, Manish Kumar, Neelabh Srivastava, Anadi Krishna Atul
When the dimensions of semiconductors are reduced to a nanoscale, their physical properties change significantly. Photoluminescence, unlike XRD, IR, and Raman spectroscopy, is very sensitive to the surface effects or adsorbed species of semiconductor particles. So, PL can be used as a probe of electron-hole surface processes and to study compound semiconductor surfaces. For device fabrication, using semiconductors of smaller dimensions, such studies help check the feasibility of improved technology. PL spectroscopy provides information about the surface state density by intensity variations and width of the spectrum. Surface states are due to the interruption of the periodic arrangement of the atoms or to the deposition of impurities at the surface. This effect is more pronounced in nanoparticles due to their large surface-to-volume ratio. Figure 13.11b shows the PL spectrum of ZnO nanoparticles having an excitation wavelength of 320 nm at RT [53], which depicts two emission peaks at ~ 392 nm (UV region) and around 520 nm. These peaks are identified due to the near bandgap exciton emission and the existence of singly ionized oxygen vacancies, respectively [53]. This emission is caused by the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy [53]. The PL spectrum also shows the narrow size distribution of nanoparticles in the ZnO powder as the full width half maximum (FWHM) of the luminescence peak is only in few nanometers.
Tunneling Spectroscopy
Published in Arthur T. Hubbard, The Handbook of Surface Imaging and Visualization, 2022
When dl/dV behavior is measured at lower tip-sample biases, the structure in the dl/dV versus V curves can be associated with the surface local density of states. Structure in the surface local density of states can be a result of the bulk band structure or of true surface states arising from termination of the bulk periodicity and surface reconstructions. One disadvantage of using a constant average tunneling current is that the value of dl/dV diverges like 1/V as the V approaches zero. The result is the desired dl/dV information is superimposed on a large background signal. Also to maintain the tunneling current the tip closely approaches the surface at low biases and in some cases may crash into the surface. This is particularly a problem when scanning a semiconductor that has a very low density of states in the band gap. Because the electronic structure near the Fermi level of a semiconductor is often of interest, this is a severe limitation. In practice, useful data is not obtained at biases of less than 1 V.
Fundamental Phenomena in Nanoscale Semiconductor Devices
Published in Ashish Raman, Deep Shekhar, Naveen Kumar, Sub-Micron Semiconductor Devices, 2022
Zeinab Ramezani, Arash Ahmadivand
The electronic states that emerge at the surface of materials are surface states. Regarding the sharp transition from a solid material, which ends with a surface, surface states are formed and only found at the atomic layers close to the surface. They may occur between VB and CB by typical energies inside the gap. They are generally partly filled; thus, the chemical potential is located through the surface band. Thereby, bending of the energy bands happens, and the FL becomes pinned, which is of utmost importance for semiconductor heterostructures. To gain energies and wave functions, Schrödinger's equation should be solved straightforwardly in a realistic potential.
Magnetic-field-induced surface quantum states in organic conductors
Published in Philosophical Magazine, 2020
At the surface of solids new electron states may develop due to change in periodicity of the crystal lattice that are not present in the bulk of the solid. These electron states known as surface states are different from the bulk electron states since they are related to the change in local electronic structure of the material. Experimentally, the surface states are generally investigated best by the techniques of angle-resolved photoemission and inverse photoemission [1–4]. Application of an external magnetic field leads to an inhomogeneity in the distribution of the electrons in the surface layer of the metal, and influences the conditions of their motion, particularly their interaction with the surface, thereby increasing the probability of specular reflection. In a bounded solid, the surface electrons are reflected many times from the boundary and drift along the surface contrary to the bulk electrons which are not affected by the presence of the boundary.
Electronic properties and surface reactivity of SrO-terminated SrTiO3 and SrO-terminated iron-doped SrTiO3
Published in Science and Technology of Advanced Materials, 2018
Aleksandar Staykov, Helena Tellez, John Druce, Ji Wu, Tatsumi Ishihara, John Kilner
The calculations of the bulk electronic properties provide important information for the materials, such as electronic and ionic conductivities, photoelectric properties, optical properties, thermal conductivities, etc., however, they provide only limited information for the surface-related properties, such as catalytic activity for various reactions. In order to understand those processes, we should investigate the electronic properties to several surface layers of the material. Surface states differ from bulk electronic structure by providing additional electron density through dangling bonds or surface reconstruction. In addition, depending on the surface termination, some bands might be missing from the surface spectrum. DOS plots and PDOS plots for SrO terminated slabs of SrTiO3, iron doped SrTiO3 without oxygen vacancies, and iron doped SrTiO3 with one oxygen vacancy per two iron atoms (vacancy on the surface and within the subsurface layer) are shown in Figure 3. Figure 3(A) plots the total DOS of the investigated surface slab of SrTiO3 and the PDOS of the surface (only) oxygen atoms and surface (only) Sr atoms. The plots demonstrate that surface oxygen atoms contribute to the valence band but there is no contribution to the conduction band. Sr atoms do not have any significant contribution either to the valence band or to the conduction band as their occupied energy levels are characterized with lower energies and their non-occupied levels are characterized with higher energies. Figure 3(B) plots the total DOS of the investigated surface slab of iron doped SrTiO3 without oxygen vacancies and the PDOS of the surface (only) oxygen atoms and surface (only) Sr atoms. While the slab DOS resembles the bulk DOS plotted in Figure 1(B), the SrO surface termination is characterized only with states from the valence band, while the conduction band is not presented on the surface. Same conclusions can be made for iron doped SrTiO3 with oxygen vacancies in the surface and subsurface layers plotted in Figure 3(C) and (D), respectively. All investigated slabs are characterized with surface states only from the valence band, while the lower energy edge of conduction band does not propagate to the surface.