Strain-engineered photoluminescence of silicon nanoclusters

Abstract

Density functional calculations on silicon clusters show that strain effects on the band gap display qualitatively new trends for dots smaller than ϳ2 nm. While the bulk indirect band gap increases linearly with increasing strain, this trend is reversed for small clusters ͑ഛ1 nm͒. In the intermediate 1 -2 nm size range, strain appears to have almost no effect. These results follow from the fact that the bonding/antibonding character of the HOMO and the LUMO change nonmonotonically with size. Since the strain level of the surface atoms dominate this behavior, they strongly stress the role of surface passivation on experimentally measured band gaps.The decrease in size of semiconductor nanoclusters can result in a variety of new properties that are absent in the bulk. For example, while bulk silicon does not emit visible light, photoluminescence is found for clusters smaller than a critical dimension. 1-4 This effect is ascribed to strong quantum confinement. In addition, the band gap in silicon quantum dots has been shown to substantially vary through surface passivation, 5-8 i.e., oxygen and other double bonded atoms have a dramatic effect as compared to hydrogenated clusters. Passivation with single bonded atoms, however, has minimal impact. These findings have led to the possibility of using silicon dots in technological applications ranging from optoelectronic devices to biological labels. 9,10 Several experiments have studied the role of strain and quantum confinement on optical emission. Examples include strained silicon dots ͑with size larger than 2 nm͒ embedded in a silicon dioxide matrix, 11,12 and InAs dots embedded in In x Ga 1−x As confining layers: the strain in this case is controlled by varying the In composition of the confining layers. 13,14 In addition, Buda and Aparisi et al. 15,16 studied the electronic and optical properties of III-V and II-VI semiconductor clusters encapsulated in the zeolites ͑and sodalite͒ cage and observed the shift of the energy gap of the semiconductor clusters due to resultant tensile/compressive strain exerted by the cage on the clusters. While these works find evident strain effects on the band gap, to our knowledge no theoretical study has focused on the coupled effects of size and strain in the case of semiconductor nanodots. This work bridges this gap, especially focusing on the sub-2 nm size range, where we predict that new, interesting phenomena will occur.We have studied the energy gap ͑EG͒ of silicon clusters containing up to Si 123 H 100 ͑ϳ1.7 nm diameter͒ using several computational tools. The starting atomic coordinates for these clusters are obtained from bulk silicon ͑lattice constant of 0.5461 nm͒. The dangling bonds of the surface silicon atoms are terminated by hydrogen at an initial bond length of 0.147 nm. The structure of the cluster is then relaxed using the energy minimization technique. The atomic and electronic structure of these clusters are computed via density functional theory ͑DFT͒ based on the generalized gradient approximation ͑GGA͒ using ...

Strain-engineered direct-indirect band gap transition and its mechanism in two-dimensional phosphorene

“…This linear shift of energy is not unique to phosphorene. It is also observed in other semiconducting nanostructures [28,29,[31][32][33][34][35][36][37][38][51][52][53][54].…”

“…This mechanism has been applied successfully in many other semiconductor nanostructures [28,29,[31][32][33][34][35][36][37][38]. Effective masses of charge carriers (thus carrier mobility) were also found to be drastically tuned by strain.…”

“…However, the present work is mainly focused on the strain effect on the band structure, and the advanced hybrid functional method HSE06 gives results of the strain effects on the electronic band structures consistent with that of the DFT-PBE method. In addition, previous studies [32] on Si nanoclusters showed that the energy gap calculated by DFT obeys a similar strain-dependency as the optical gap predicted by the advanced configuration interaction method and the quasiparticle gap. Therefore, we expect that the DFT can correctly predict the general trends of strain effect on the band structure and near-gap states in phosphorene.…”

Engineering direct-indirect band gap transition in wurtzite GaAs nanowires through size and uniaxial strain

“…For state B, the anti-bonding characteristics makes the Coulomb energy between the electron and the nuclei less sensitive to uniaxial strain. 29 However, the anti-bonding characteristics suggests that the nodal surfaces of the positive and negative values of the wavefunction are perpendicular to the z-direction and a compressive uniaxial strain reduces the distance between the nodal surfaces, therefore kinetic energy associated with the electron transportation between atoms increases. [30][31][32] In contrast, a tensile strain increases the nodal surfaces thus reducing the associated kinetic energy.…”

First Principles Study of Si/Ge Core-Shell Nanowires ---- Structural and Electronic Properties

“…The present work is mainly focused on the variation of electronic properties under factors such as external strain and size. Previous studies (Peng et al, 2006) on Si nanoclusters showed that the energy gap calculated by DFT obeys a similar straindependency as the optical gap predicted by advanced configuration interaction (CI) method and the quasi-particle gap (defined as the difference of ionization potential and electron affinity). In addition, DFT gap predicts a similar size-dependency as the optical gap obtained using GW and quantum Monte Carlo methods (Puzder et al, 2002;Zhao et al, 2004).…”