Size, Composition, Shape And Pressure Effects On The Bandgap Of Nano-semiconductors

In recent years, materials within nanoscale became the study issue in modern science and technology fields due to their fascinating physical, chemical and mechanical properties that are different from the corresponding bulk counterparts. As an important property for designing optoelectronic devices, bandgap Eg plays an important role on the final performance of the device. Three simple and unified models have been established to predict bandgap. The theoretical prediction agrees approximately with experimental and computer simulation results of semiconductor Nanocrystals. Major progress are followed:(1) Based on a thermodynamic model for size-dependent melting temperature, a simple model has been established for size and composition dependent bandgap by considering the interaction force between atoms. It predicts an increase of the bandgap of the nano-semiconductor alloy with the decrease of the size. The bandgap is an approximate linear function of size when the size is less than 5 nm. And bandgap becomes a nonlinear function with the increase of the size. The accuracy of model prediction is confirmed by the experimental and simulation results.(2) Based on size-dependent melting temperature model, a quantitative thermodynamic model without any adjustable parameter was established by considering the variation of surface-to-volume ratio. It is found that the bandgap of nanorod increases with the decreasing of size and length where the size confinement is the principle factor and the length,confinement is the secondary one. In addition,for nanocrystals with different shapes, the size effect on bandgap can be sequenced as:Eg(D, L) >Eg(D) nanoparticles > Eg(D) nanowires at L < D, while Eg(D)nanoparticles > Eg(D, L) > Eg(D) nanowires at L > D. The model prediction agrees well with the experimental results of CdSe nanorods.(3) A simple thermodynamic model is developed and the competition relation among size, dimensionality and pressure effects on Eg is discussed. The change of Eg(D,d,P) for nanoparticles and nanowires occurs as D below 5 nm, and an abruptrise tendency is present. The size and dimensionality effect is directly related to the A/V ratio, which can be expressed by A/V = 6/D, A/V = 4/D and A/V = 2/D for nanoparticles, nanowires and nanofilms respectively. The pressure induced additional energy on nanocrystals is mainly executed on each bond. As known that A/V can be expressed as 6/D for nanoparticles, among which nanoparticles have the largest surface curvature, broken bonds number of surface layer, and then the largest bond contraction or the strongest bond energy, while nanofilms represent the reverse cases.