The nanostructures have been studied, mainly involving nanoparticle, nanowire and nanolayer in the past decade [1-5]. Especially, scientists have made an attention to the nanolayer (NL) structure due to its unique properties and applications [6–13]. Recently, in the quantum system of Si NL, some interesting phenomena and new effects have been discovered. In last few years, scientists have invested strong effort to growth of 2D silicon material [14, 15], which was expected to have a great impact on the development of future electronic devices and energy storage [16-18]. The molecular dynamics simulation was used to study the formation of double-layer silicon in slit pores, where their stability is further confirmed by first principles calculation in the simulated calculation [19, 20].
Here, it is interesting that the quantum confinement (QC) effect play a main role as the NL thickness is larger than 0.5nm. The energy bandgap obviously increases with decreasing thickness of NL by the QC effect in the quantum system of NL on silicon. In the article, the QC effect has been exhibited as the thickness of Si NL changes along with (100), (110) and (111) direction respectively in simulating calculation. And the simulation result demonstrated that the direct bandgap can be obtained as the NL diameter is smaller than 10nm in NL with (001) direction on Si surface. However, the bandgap energy of the NL changes in complicated ruler as the NL reaches to 2D Si material, where a new effect takes place on the energy change of NL bandgap. The bandgap energy in quantum system of NL on silicon surface abruptly changes as the NL is near 2D structure. It is discovered in the simulated calculation that the QC effect disappears as the NL thickness arrives at size of monoatomic layer, in which its bandgap energy sharply deceases where the abrupt change effect of bandgap energy occurs.
Fabrication of silicon nanolayer
Scientists usually used the self-assembly ways from silicon-rich silicon oxide matrices and plasma synthesis methods to fabricate various silicon nanostructures [21–27]. Here, the interesting method for fabricating silicon nanocrystal is growth under laser photons interaction [28–30]. We have taken the interesting and simplest method for fabricating silicon nanolayer (NL), in which Si NL crystal rapidly grows with irradiation of electron beam on amorphous silicon film prepared by using pulsed laser deposition (PLD). The novel method of electron affection could be used to replace the traditional annealing methods in preparing process of silicon nanocrystals [31].
The coherent electron beam with 0.5 nA/nm2 was used to irradiate on amorphous silicon film for 10-30min in Tecnai G2 F20 system, in which the electron beam from field-emission electron gun was accelerated by 200 KV and had higher energy and better coherent, where the silicon nanolayers with various thickness rapidly grew. The TEM image exhibits a geometry top viewed on the Si nanolayer as shown in Fig.1. Truly interesting, the several structures of silicene with quasi-2D film were obtained through controlling irradiation time and density of the coherent electron beam, which are respectively related to growing along with (100), (110) and (111) direction.
Investigation on Si nanolayer in simulation
The dynamic stability of the nanolayer was investigated by using first-principles calculations with consideration of three kinds of crystal structures observed in experiment, in which the crystal lattice grows respectively along with (100), (110) and (111) direction. The electronic behavior on the Si nanolayer was investigated by an ab initio nonrelativistic quantum mechanical analysis. The density functional theory (DFT) was used to calculate the density of states (DOS) on silicon nanolayers, which is carried out with the local density approximation (LDA) and gradient-corrected exchange-correlation function (GGA) for the self-consistent total energy methods.
It is interesting in the total energy calculations using DFT that the transformation from the indirect bandgap to direct bandgap can be obtained as the nanolayer thickness is smaller than 10nm, wholes simulated model of NL structure along with (001) direction and its direct bandgap are shown in Fig.2(a) and (b) after simulated calculation. It is originated from the Heisenberg principle related to ⊿hKx ~ h/⊿x, in which the wave vector will relax from X region to Γ region as the ⊿x decreases to nanoscale (relaxing relation: ⊿x↓ →⊿Kx↑), where higher electronic speed is obtained in nanolayer.
In the experimental result, the nanolayer of silicon crystal growing along with (100) direction was prepared, as shown in Fig.3(a), where the TEM image exhibits its structure. In the simulated calculation, the DMol3 mode is used to make optimum atomic structure for obtaining the lowest combining energy, and the CASTEP mode is used to simulate for obtaining the energy band structure after optimum process. In Fig.3(b), the simulation model of the Si nanolayer has been built along with (100) direction according to the experimental result. After optimization process in simulation, the quasi-2D structure of silicon crystal with the rectangular lattice occurs in the lowest energy of optimum structure, as shown in Fig.3(c). The structural transformation from the ideal model of 2D structures to the optimal quasi-2D structures of Si crystal along with (100) direction should be noted in the simulation process, where the convex bonding angles take place on silicon film.
The Fig.3(d) exhibits the change curve of bandgap energy with various thicknesses of Si NL along with (100) direction in the simulation result, where it is interesting that the quantum confinement (QC) effect plays a main role as the nanolayer thickness is lager, but the QC effect disappears as the nanolayer thickness is smaller than 0.4nm near monoatomic layer, when its bandgap energy abruptly decreases. Truly interesting, the QC effect disappears as the nanolayer thickness reaches to size of monoatomic layer, where its bandgap sharply deceases. This abrupt change effect in bandgap energy may be originated from transforming between different dimensions at the symmetry broken point, in which the quasi-2D shape of NL is transformed to the two dimensional quantum layer. Here, the abrupt change effect in bandgap energy was observed and studied at first.
The nanolayer structure of the Si crystal growing along with (110) direction was obtained, as shown in the TEM image of Fig.4(a), from whom we can build the simulation model of the Si nanolayer in this direction, as exhibited in Fig.4(b). The idea model structure with the hexagonal lattice is transformed to the real quasi-2D structure with convex atomic bonds after simulated optimum process, as shown in Fig.4(c). In the same way, the abrupt change effect in bandgap energy takes place as the nanolayer thickness reaches to size of monoatomic layer, where the QC effect disappears, as shown in Fig.4(d). Here, it is interesting to make a compression between the quasi-2D silicene with the hexagonal lattice and the graphite. The simulation result demonstrates that the indirect bandgap can be transformed to the direct bandgap as the nanolayer thickness along with (110) direction is smaller than 3nm.
In the same way, the TEM image in Fig.5(a) shows the nanolayer structure of the Si crystal growing with (111) direction, according to whom the simulated model of the Si nanolayer in this direction can be built, as shown in Fig.5(b). The real quasi-2D structure along with Si crystal (111) direction can be obtained after simulated optimum process, as exhibited in Fig.5(c). The result of simulated calculation demonstrates that the QC effect plays a main role as the nanolayer thickness is larger, in which the indirect bandgap is almost kept in thickness changing. In the same manner, the abrupt change effect in bandgap energy was discovered in the simulated calculation, where the bandgap energy deeply decreases as the nanolayer thickness arrives near monoatomic layer, as shown in Fig.5(d). Here, it is should be noted that the bandgap rapidly disappears in the process.
The abrupt change effect in bandgap energy may be originated from transforming between different dimensions at the symmetry broken point. In the picture (c) of Fig.3, Fig.4 or Fig.5, the structure near monoatomic layer belongs to the fractional dimension of 2.1~2.5 in quasi-2D situation, and the bandgap will disappear to become a semi-metal, where new quantum phenomena and effects will appear in the process from the 3D layer to the quasi-2D shape and to the idea quantum surface.