Jun Zhuang

A nitrogen-hyperdoped silicon material formed by femtosecond laser irradiation

A supersaturation of nitrogen atoms is found in the surface layer of microstructured silicon after femtosecond (fs) laser irradiation in NF3. The average nitrogen concentration in the uppermost 50 nm is about 0.5 ± 0.2 at. %, several orders of magnitude higher than the solid solubility of nitrogen atoms in silicon. The nitrogen-hyperdoped silicon shows high crystallinity in the doped layer, which is due to the repairing effect of nitrogen on defects in silicon lattices. Nitrogen atoms and vacancies can be combined into thermal stable complexes after fs laser irradiation, which makes the nitrogen-hyperdoped silicon exhibit good thermal stability of optical properties.

Physical mechanisms for the unique optical properties of chalcogen-hyperdoped silicon

A series of fundamental properties from atomic geometry, electronic band structure, optical absorption, to dynamics are systemically studied for the silicon doped with supersaturated chalcogens (S, Se, and Te). The atomic structures in a broad energy range are obtained and distinguished as three classes named the substitutional, interstitial, and quasi-substitutional. Their relative energies varying with the S, Se, and Te samples reveal that the concentration of impurity atoms occupying the substitutional position, which plays an important role in optical absorption, will be different and get progressively higher from S-, Se-, to Te-hyperdoped silicon. Electronic band structures show that for the most atomic geometries the defect-related states do appear into the gap of silicon, and the optical absorption calculations clarify that they are the very origin of the broadband absorption of chalcogen-hyperdoped silicon. Combining the optical absorption properties with the structural transformation from molecular-dynamics simulations, we disclose the micromechanism of annealing-induced reduction of infrared absorptance. Furthermore, we conclude that both the different concentrations of the substitutional doping and structural transformation will lead to the different annealing-induced reduction of infrared absorptance for S-, Se-, and Te-hyperdoped silicon as observed in experiments.

Mechanisms for adatoms diffusing on metal fcc(111) surfaces

Abstract On a series of fcc(111) surfaces modeled by the embedded atom method, adatom diffusion mechanisms are studied systematically by molecular dynamics. On most surfaces, as expected, the diffusion mechanism is only hopping. For some other surfaces, however, we find that the exchange mechanism is possible besides the well-known hopping mechanism. More interesting is that the hcp and fcc sites are now different for adatom diffusion: the exchange diffusion process only occurs between the hcp sites.

Statistical model for small clusters transforming from one isomer to another

Based on the fact that the kinetic energy of one atom in small cluster still obeys the Boltzmann distribution, a statistical model is developed to predict the time consumed by a small cluster transforming from one isomer to another and is tested by vast molecular dynamics simulations of C(12) isomers transformation in helium gas at high temperatures (2000-3500 K). Extrapolating the model to lower temperatures, we found that the time for the most probable isomer of C(12) formed at 2500 K turning into the most stable one is more than 10(12) years at room temperature.

Hybrid Functional Studies on Impurity-Concentration-Controlled Band Engineering of Chalcogen-Hyperdoped Silicon

We employ the hybrid functional method to study the impurity-concentration-controlled band engineering of chalcogen-hyperdoped silicon. The variations of defect band width and distance to the conduction band with increasing impurity concentration are given, in which at very low concentration, the reasonable depths of defect states and the energy gap compared with the experiments indicate the good accuracy of hybrid functional calculations for the band structures. For S- and Se-doped silicon, the critical concentrations for the merging of defect and conduction bands are obtained. At the key concentrations related to the insulator-to-metal transition, the characteristics of band structures are discussed.

Formation of various types of metallofullerenes by laser ablation of externally doped fullerenes C60Mx (M=Sm, Pt, Ni, La, Y, and Rh)

A photofragmentation study of metal fullerides C60Mx (M=Sm, Pt, Ni, La, Y, and Rh) by excimer laser ablation-TOF mass spectrometry shows that many kinds of metallofullerenes have been observed in both the positive and negative ionic modes. For C60Mx (M=Sm, Pt, and Ni), the metal atom is incorporated into the network of the fullerene cage to replace one carbon atom of the cage forming substitutional metallofullerene. While in the case of metal fullerides C60Mx (M=La, Y), evidence of the encapsulation of La and Y atoms in fullerene cages forming endohedral fullerenes has been observed. Different from the above two cases, the laser ablation of rhodium fulleride C60Rhx(x≈1) produces two sequences of rhodium-containing fullerene clusters C2nRh and C2n+1Rh(2n=50–62). Odd-numbered all-carbon clusters in the fullerene regime are observed in our laser ablation study of all the metal fullerides in the negative ion channel. The structures of metallofullerenes C2n+1M and C2nM with an even and odd number of total atom...

Reliable lateral and vertical manipulations of a single Cu adatom on a Cu(111) surface with multi-atom apex tip: semiempirical and first-principles simulations

We study the reliability of the lateral manipulation of a single Cu adatom on a Cu(111) surface with single-atom, dimer and trimer apex tips using both semiempirical and first-principles simulations. The dependence of the manipulation reliability on tip height is investigated. For the single-atom apex tip the manipulation reliability increases monotonically with decreasing tip height. For the dimer and trimer apex tips the manipulation reliability is greatly improved compared to that for the single-atom apex tip over a certain tip-height range. Two kinds of mechanism are found responsible for this improvement. One is the so-called enhanced interaction mechanism in which the lateral tip-adatom interaction in the manipulation direction is improved. The other is the suspended atom mechanism in which the relative lateral trapping ability of the tip is improved due to the strong vertical attraction of the tip on the adatom. Both mechanisms occur in the manipulations with the trimer apex tip, while in those with the dimer apex tip only the former is effective. Moreover, we present a method to realize reversible vertical manipulation of a single atom on a Cu(111) surface with the trimer apex tip, based on its strong vertical and lateral attraction on the adatom.

Electronic Band Structure and Sub-band-gap Absorption of Nitrogen Hyperdoped Silicon

We investigated the atomic geometry, electronic band structure, and optical absorption of nitrogen hyperdoped silicon based on first-principles calculations. The results show that all the paired nitrogen defects we studied do not introduce intermediate band, while most of single nitrogen defects can introduce intermediate band in the gap. Considering the stability of the single defects and the rapid resolidification following the laser melting process in our sample preparation method, we conclude that the substitutional nitrogen defect, whose fraction was tiny and could be neglected before, should have considerable fraction in the hyperdoped silicon and results in the visible sub-band-gap absorption as observed in the experiment. Furthermore, our calculations show that the substitutional nitrogen defect has good stability, which could be one of the reasons why the sub-band-gap absorptance remains almost unchanged after annealing.

Shape prediction of two-dimensional adatom islands on crystal surfaces during homoepitaxial growth

A dynamic model based on surface interaction potentials is established to predict the geometric structure of two-dimensional (2D) adatom islands on surfaces formed at temperatures for general epitaxial growth. The prediction based on the model is independent of the number of atoms in the island and can be implemented easily by ab initio calculations. Predictions of the homoepitaxial 2D islands on Pt(111), Ag(111), and Cu(111) surfaces are in good agreement with relevant experimental observations.

Formation of odd-numbered fullerene-related species and its relation to the formation of metallofullerenes

Abstract Odd-numbered carbon clusters in the fullerene regime are formed through laser ablation of various fullerene-containing materials: organic fullerene derivatives, metal fullerides, and guest-host fullerene materials. The formation mechanism of odd-numbered “fullerene” species is briefly discussed. The structural properties are calculated with genetic algorithm and ab initio methods, and the results show that the odd-numbered fullerene species are pseudo-open-caged fullerenes. In the laser ablation of metal fullerides, substitutional or endohedral metallofullerenes are observed. A new way for metallofullerene preparation through the odd-numbered fullerene intermediates is suggested and realized experimentally.