Ji-Hai Liao

The nucleation and growth of borophene on the Ag (111) surface

B sheets have been intently studied, and various candidates with vacancies have been reported in theoretical investigations, including their possible growth on metal surfaces. However, a recent experiment reported that the borophene formed on a Ag (111) surface consisted of a buckled triangular lattice without vacancies. Our calculations propose a novel nucleation mechanism of B clusters and emphasize the B–Ag interaction in the growth process of borophene, demonstrating the structural evolution of triangular fragments with various profiles and vacancy distributions. Compared with the triangular lattice without vacancies, we have confirmed that the sheet energetically favored during the nucleation and growth is that containing 1/6 vacancies in a stripe pattern, whose scanning tunneling microscopy image is in better agreement with the experimental observation.Boron (B) sheet has been intently studied and various candidates with vacancies have been proposed by theoretical investigations, including the possible growth on metal surface. However, a recent experiment (Science 350, 1513, 2015) reported that the sheet formed on the Ag(111) surface was a buckled triangular lattice without vacancy. Our calculations combined with High-Throughput screening and the first-principles method demonstrate a novel growth mechanism of boron sheet from clusters, ribbons, to monolayers, where the B-Ag interaction is dominant in the nucleation of boron nanostructures. We have found that the simulated STM image of the sheet with 1/6 vacancies in a stripe pattern is in better agreement with the experimental observation, which is energetically favored during the nucleation and growth.

Understanding the stable boron clusters: A bond model and first-principles calculations based on high-throughput screening.

The unique electronic property induced diversified structure of boron (B) cluster has attracted much interest from experimentalists and theorists. B30-40 were reported to be planar fragments of triangular lattice with proper concentrations of vacancies recently. Here, we have performed high-throughput screening for possible B clusters through the first-principles calculations, including various shapes and distributions of vacancies. As a result, we have determined the structures of Bn clusters with n = 30-51 and found a stable planar cluster of B49 with a double-hexagon vacancy. Considering the 8-electron rule and the electron delocalization, a concise model for the distribution of the 2c-2e and 3c-2e bonds has been proposed to explain the stability of B planar clusters, as well as the reported B cages.

Two-Dimensional Semiconducting Boron Monolayers

The two-dimensional boron monolayers were reported to be metallic both in previous theoretical predictions and experimental observations. Unexpectedly, we have first found a family of boron monolayers with the novel semiconducting property as confirmed by the first-principles calculations with the quasi-particle G0W0 approach. We demonstrate that the connected network of hexagonal vacancies dominates the gap opening for both the in-plane s+px,y and pz orbitals, with which various semiconducting boron monolayers are designed to realize the band gap engineering for the potential applications in electronic devices. The semiconducting boron monolayers in our predictions are expected to be synthesized on the proper substrates, due to the similar stabilities to the ones observed experimentally.

Theoretical investigations on diamondoids (CnHm, n = 10–41): Nomenclature, structural stabilities, and gap distributions

Combining the congruence check and the first-principles calculations, we have systematically investigated the structural stabilities and gap distributions of possible diamondoids (CnHm) with the carbon numbers (n) from 10 to 41. A simple method for the nomenclature is proposed, which can be used to distinguish and screen the candidates with high efficiency. Different from previous theoretical studies, the possible diamondoids can be enumerated according to our nomenclature, without any pre-determination from experiments. The structural stabilities and electronic properties have been studied by density functional based tight binding and first-principles methods, where a nearly linear correlation is found between the energy gaps obtained by these two methods. According to the formation energy of structures, we have determined the stable configurations as a function of chemical potential. The maximum and minimum energy gaps are found to be dominated by the shape of diamondoids for clusters with a given number of carbon atoms, while the gap decreases in general as the size increases due to the quantum confinement.

An electron compensation mechanism for the polymorphism of boron monolayers

Boron monolayers have been increasingly attractive, while it is still a challenge to understand their structural stabilities, due to electron deficiency and multi-center bonds. In this work, we propose the average electron compensation (AEC) mechanism for boron monolayers based on high-throughput first-principles calculations. It is found that the AEC parameter (λ) tends to be zero for the stable free-standing boron monolayers. In addition, this mechanism can quantitatively describe the stability of boron monolayers on various metal substrates, providing direct suggestions for experimentalists to synthesize various boron monolayers for practical applications.

An extended cluster expansion for ground states of heterofullerenes

It is challenging to determine the ground states of heterofullerenes due to the numerous isomers. Taking the C60-nBn heterofullerenes (1 ≤ n ≤ 4) as an example, our first-principles calculations with the isomer enumeration present the most stable structure of C57B3, which is energetically favored by 0.73 eV than the reported counterpart. It was difficult to conduct the enumeration for the isomers with n beyond 4 because of the expensive first-principle calculations. Here, we propose a nomenclature to enhance structural recognition and adopt an extended cluster expansion to describe the structural stabilities, in which the energies of the heterofullerenes with various concentrations are predicted by linear combination of the multi-body interactions. Unlike the conventional cluster expansion, the interaction parameters are derived from the enumeration of C60-nBn (n = 1~4), where there are only 4 coefficients to be fitted as a function of composition for the consideration of local bonding. The cross-validation scores are 1~2 meV per atom for both C55B5 and C54B6, ensuring the ground states obtained from our model are in line with the first-principles results. With the help of the structural recognition, the extended cluster expansion could be further applied to other binary systems as an effective complement to the first-principle calculations.

Gap maximum of graphene nanoflakes: a first-principles study combined with the Monte Carlo tree search method

The energy gap of graphene nanoflakes is important for their potential application in nano-devices; however, it is still a challenge to perform a systemic search of systems with large gaps due to the presence of numerous candidates. Herein, we showed an ideal feasible approach that involved structural recognition, simplified effective evaluation, and successive optimization strategy. Considering the local bonding environment of carbon atoms, we first proposed a tight-binding model with the parameters fitted from the first-principles calculations of possible GNFs; this model provided an ideal avenue to screen the candidates with high accuracy and efficiency. Via combining the Monte Carlo tree search method and the congruence check, we determined the correlation between structures and the gap distributions according to the carbon numbers, and the results were confirmed via the first-principles calculations. The structural stabilities of the candidates with different numbers of hydrogen atoms might be modulated by the chemical potential of hydrogen, whereas the candidates with larger gaps might be more stable for the isomers with the same number of C and H atoms. Note that the gap variation is dominated by the structural features despite the quantum confinement effect since the gap maximum fluctuates rather than gradually decreasing with the increase in size. Our finding shows the gap variety of GNFs due to the configuration diversity, which may help explore the potential application of GNFs in nano-devices and fluorescence labeling in biomedicine.

Temperature effect on the structural stabilities and electronic properties of X22H28 (X=C, Si and Ge) nanocrystals: A first-principles study

Based on ab initio molecular dynamic simulations, we have theoretically investigated the structural stabilities and electronic properties of X22H28 (X=C, Si, and Ge) nanocrystals, as a function of temperature with consideration of vibrational entropy effects. To compare the relative stabilities of X22H28 isomers, the vibration free energies are obtained according to the calculated phonon spectrum, where the typical modes are shown to be dominant to the structural stabilities. In addition, there is a significant gap reduction as the temperature increases from 0 K to 300 K, where the decrements are 0.2 /0.5 /0.6eV for C/Si/Ge nanocrystals, respectively. The dependence of energy gap on the variance of bond length is also analyzed according to the corresponding atomic attributions to the HOMO and LUMO levels.