Quantum Confinement of Crystalline Silicon Nanotubes

1
Quantum Confinement of Crystalline Silicon
Nanotubes
Binghai Yan1, Gang Zhou1, Xiao Cheng Zeng2,
Bing-Lin Gu1, and Wenhui Duan1 1Department of
Physics,Tsinghua University, Beijing
100084,China2Departments of Chemistry and
Physics, University of Nebraska, Lincoln, NE
68588,USA
Crystalline silicon nanotubes(c-SiNTs),
synthesized in experiments recently, are
nanotubes of Si crystalline structure with
tube-wall-thickness of several nanometers, which
are expect to have more advantages to exhibit
quantum confinement effect and potential
applications than silicon nanowires(SiNWs).
Our work studied the electronic, optical and
quantum confinement properties of c-SiNTs with
both uniform thickness and non-uniform thickness
from first-principles.
Method
Thickness(s)

• Density-functional theory.
• The local density approximation.
• Norm-conserving pseudopotentials.
• Numerical atomic orbitals.
• SIESTA package.

Diameter
TEM image of c-SiNT in experiments (left) and
structural model of c-SiNT along 111 lateral
view (middle) and cross section (right).
Results
Uniform thickness c-SiNTs. The band structure of
nanotubes with D1.80 nm and diffrenet thickness.
The band gap increases significantly as the
thickness becomes smaller due to quantum
confinement.
Non-uniform thickness c-SiNTs. The wave-functions
of the VBM and CBM are due mainly to atoms
located in the thicker side of the tube wall,
referred as effective confinement region (ECR).
The band gap also depends mainly on ECR.
Conduction Channel
SEPARATE
FET
Dopant Impurity
For c-SiNTs with non-uniform thickness, the
non-uniformity of the VBM and CBM shows a way to
spatially separate the doping impurities from the
conducting channel in doped SiNTs. Practically,
the performance of the SiNT-based transistors may
be substantially improved by selective p/n doping
in the thinner side of the tube wall, in the
spirit of modulation doping.
For Si nanowires, the dopants will scatter the
carriers a lot because the conduction channel and
the dopants are in the same position. However,
providing the impurity scattering and increasing
the conductance is very important to improve the
performance of devices, such as FET,
Summary Our study clearly reveals the unique
electronic properties of c-SiNTs, especially the
effect of non-uniformity in wall thickness,
suggesting a new modulation doping method aiming
to high-performance nanodevices. Acknowledgement W
e thank Prof. Jisoon Ihm and Dr. Chaoxing Liu for
their helpful discussions.