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2D Material Photodetectors
Published in Antoni Rogalski, Zbigniew Bielecki, Detection of Optical Signals, 2022
Antoni Rogalski, Zbigniew Bielecki
In practical applications, the stability of materials is an important factor that affects the reliability and lifetime of a device. The electronic properties of TMD materials are mainly determined by the filling of metal atom d orbitals, while the lattice parameters and stability primarily depend on the chalcogen atom [12]. The bonding of the M–X is covalent: the metal (M) atom provides four electrons to fill the bonding states, while the lone-pair electrons of the chalcogen (X) atoms terminate the surfaces of the layers. The absence of dangling bonds reduces the chemical instability and protects the surface atoms from reacting with environmental species. Thus, notably, the more stable the lone pair of the chalcogen (X) atoms are, the more stable the 2D materials will be. This explains, for example, why monolayer MoS2 is more stable than monolayer MoTe2.
Relevant Properties of Graphene and Related 2D Materials
Published in Antoni Rogalski, 2D Materials for Infrared and Terahertz Detectors, 2020
In practical applications, the stability of a material is an important factor, affecting the reliability and lifetime of a device. The electronic properties of TMD materials are mainly determined by the filling of metal atom d orbitals, whereas the lattice parameters and stability primarily depend on the chalcogen atom. The bonding of the M–X is covalent; the metal (M) atom provides four electrons to fill the bonding states, while the lone-pair electrons of the chalcogen (X) atoms terminate the surfaces of the layers. The absence of dangling bonds reduces the chemical instability and protects the surface atoms from reacting with environmental species. Thus, the more stable the lone-pair electrons of the chalcogen (X) atoms are, the more stable the 2D materials will be. This explains, for example, why monolayer MoS2 is more stable than monolayer MoTe2 (Fig. 5.7).
Contacting Systems
Published in Alan Owens, Semiconductor Radiation Detectors, 2019
It was soon found experimentally (see for example, Schweikert [18]) that barrier heights in “real” metal-semiconductor systems vary proportionally with the metal work function, but with a slope much smaller than unity, typically between 0.1 and 0.3. The departure of experiment from theory was first explained in terms of localized electronic surface states [19] or “dangling bonds” resulting from immobilized atoms with unfulfilled valence [20,21] (i.e., broken or missing bonds). The dangling bond may have an unpaired electron, two unpaired electrons or no unpaired electron and arises naturally at the surface of a solid because the atoms have neighbours on one side only. As a consequence, at a metal-semiconductor interface the wave functions of the metal electrons decay exponentially into the semiconductor forming a continuum of metal-induced gap states (MIGS) in the bandgap [22]. Physically, these states may extend up to one nm into the semiconductor and determine the barrier height in an ideal, abrupt, defect-free and laterally homogenous metal-semiconductor contact. However, in practice, there is limited agreement between experiment and theory, since surfaces are rarely ideal, because of defects introduced during interface formation [23], surface contamination and oxide layers introduced during chemical processing.
Structural and electrical properties of CdS nanocomposite solid films for electrolyte applications
Published in Soft Materials, 2022
The same trend was observed for the different concentrations of CdS:PEO electrolytes (after H2S treatment). In CdS:PEO electrolytes, the confinement of charge carriers in small semiconductor particles has some influence on the conductivity of the material. Maximum value of conductivity is found for CdS:PEO nanocomposite of 1:300 concentration, in which the conductivity increases and shows a jump by two orders of magnitude and is around 2 × 10−10 S/cm. For CdS:PEO electrolytes of 1:100 concentration, it again slightly decreases with increase in CdS concentration. Venkatesu et al[18] discussed the electrical conductivity of semiconducting nano materials by considering the defect states. The density of such defects leads to the formation of dangling bonds and such defects acts as trapped states between conduction and valence band. These are all the causes which affects the dc conductivity in 1:100 concentration, even though the CdS concentration is high. In 1:300 concentration of polymer electrolyte, the conductivity value is maximum before and after H2S treatment. This can be explained by an empirical equation.
Role of thickness and annealing temperature on the structural ordering in vacuum evaporated antimony selenide thin films
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
M. Malligavathy Rajakumar, R. T. Ananth Kumar, D. Pathinettam Padiyan
The optical energy decreases with the increase in film thickness can be explained by using the density of states model, which suggests that the width of the localized states near the mobility edges are dependent on the degree of disorder and defects present in the amorphous structure (Mott and Davis 1979). The observed decrease in optical bandgap may be due to the presence of a high concentration of localized states in the band structure. In an amorphous or structurally disordered film, the imperfection causes the bands of localized states to broaden and consequently a bandgap reduction may occur with increase in film thickness (Goh et al. 2010). The insufficient number of atoms deposited in the amorphous film results in the existence of unsaturated bonds (Biswas, Chaudhuri, and Choudhury 1988). These unsaturated bonds are responsible for the formation of localized states (Arshak and Hogarth 1986). The bandgap decrease with increase in annealing temperatures for Sb2Se3 thin films can be explained as follows. When the annealing temperature increases, the width of the localized state also increases. Thus, it produces dangling bonds around the surface of the crystallites (Vishwas et al. 2010). These dangling bonds are responsible for some types of defects in the crystalline solids. As the number of dangling bonds and defects increases with annealing temperature, the concentration of localized states in the band structure increases and in turn reduces the optical gap (Kumar, Thangaraj, and Stephen Sathiaraj 2008).
A review of cutting tools for ultra-precision machining
Published in Machining Science and Technology, 2022
Ganesan G., Ganesh Malayath, Rakesh G. Mote
Thermo mechanical lapping (TMP) is a modified form of mechanical lapping where the cast iron scaife with diamond grits is replaced with a steel scaife as shown in Figure 30a. Zong et al. (2008) used TMP, where the surface layer in diamond is removed due to carbon diffusion, graphitization, oxidation, and mechanical abrasion. Here, instead of lapping time, lapping pressure and lapping velocity have a major effect. Using this method, the sharpness of the tool edge can be increased to 10 nm. The longitudinal running efficiency of the scaife spindle system should be regulated within 0.05 µm since the dynamic balancing of the spindle plays a critical role in determining the lapping accuracy. The surface quality of the scaife will also determine the polishing efficiency. When the temperature at the interface is low, diffusion of the carbon atoms is observed to be the dominant material removal mechanism. As the temperature increases, the carbon atoms gain more kinetic energy and create dangling bonds. A more stable carbon structure is formed by recombining the dangling bonds, which is known as graphitization. At a very high interface temperature, material removal from the diamond surface due to the formation of CO and CO2 is observed by Zong et al. (2008). TMP is used as a successful edge-finishing method by Jin et al. (2020) for polishing nanotwinned diamond (nt-D) (which has twice the hardness of natural diamond). Polishing with an iron scaife causes the formation of sp2-sp3 amorphous carbon layer on the nt-D. Unlike SCD and PCD, the isotropy and binderless composition help to remove the amorphous carbon layer easily and continuously while employing mechanical lapping. Figure 30b shows the cutting tool edge prepared by TMP.