Finite temperature properties of clusters by replica exchange metadynamics: the water nonamer

We introduce an approach for the accurate calculation of thermal properties of classical nanoclusters. Based on a recently developed enhanced sampling technique, replica exchange metadynamics, the method yields the true free energy of each relevant cluster structure, directly sampling its basin and measuring its occupancy in full equilibrium. All entropy sources, whether vibrational, rotational anharmonic and especially configurational -- the latter often forgotten in many cluster studies -- are automatically included. For the present demonstration we choose the water nonamer (H2O)9, an extremely simple cluster which nonetheless displays a sufficient complexity and interesting physics in its relevant structure spectrum. Within a standard TIP4P potential description of water, we find that the nonamer second relevant structure possesses a higher configurational entropy than the first, so that the two free energies surprisingly cross for increasing temperature.Comment: J. Am. Chem. Soc. 133, 2535-2540 (2011

Si3AlP: A new promising material for solar cell absorber

First-principles calculations are performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si2)0.6(AlP)0.4 alloy [T. Watkins et al., J. Am. Chem. Soc. 2011, 133, 16212.] We find that the ordered CC-Si3AlP with a basic unit of one P atom surrounded by three Si atoms and one Al atom is the most stable one within the experimentally observed unit cell.1 Si3AlP has a larger fundamental band gap and a smaller direct band gap than Si, thus it has much higher absorption in the visible light region. The calculated properties of Si3AlP suggest that it is a promising candidate for improving the performance of the existing Si-based solar cells. The understanding on the stability and band structure engineering obtained in this study is general and can be applied for future study of other nonisovalent and lattice-matched semiconductor alloys

Si₃AlP: A New Promising Material for Solar Cell Absorber

First-principles calculations were performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si₂)_0.6(AlP)_0.4 alloy (Watkins, T.; et al. <i>J. Am. Chem. Soc.</i> <b>2011</b>, <i>133</i>, 16212). We found that the most stable structure of Si₃AlP is a superlattice along the ⟨111⟩ direction with separated AlP and Si layers, which has a similar optical absorption spectrum to silicon. The ordered <i>C</i>1<i>c</i>1-Si₃AlP is found to be the most stable one among all structures with a basic unit of one P atom surrounded by three Si atoms and one Al atom, in agreement with experimental suggestions. We predict that <i>C</i>1<i>c</i>1-Si₃AlP has good optical properties, i.e., it has a larger fundamental band gap and a smaller direct band gap than Si; thus, it has much higher absorption in the visible light region. The calculated properties of Si₃AlP suggest that it is a promising candidate for improving the performance of the existing Si-based solar cells. The understanding on the stability and band structure engineering obtained in this study is general and can be applied for future study of other nonisovalent and lattice-matched semiconductor alloys