理论研究：V1-取代CsI中的V1+对CsI晶体光学性质的影响

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本文研究了CsI晶体中包含一对V缺陷时的电子结构和光学性质。采用CASTEP代码进行了计算，首先对完美CsI晶体的晶格结构进行了优化。研究发现，CsI晶体的光学对称性与其晶格结构几何相匹配，这表明其基本的光学特性受晶格规律控制。在计算过程中，作者针对含有V1-Cs-V1+I缺陷的CsI晶体进行了细致的分析。他们特别关注了这一对缺陷对光吸收谱的影响。结果显示，尽管存在这种缺陷，但CsI晶体的吸收谱并未在可见光和近紫外波段出现显著变化。这意味着这对V杂质并不会引起显著的光学吸收，从而对CsI晶体在这些波长范围内的光学性能产生明显影响。此外，这项工作还可能对理解其他材料中的杂质对光学性质的调控作用提供了启示，因为在某些情况下，杂质可能并不像预期那样显著地改变晶体的整体光学行为。对于那些依赖于纯度和缺陷控制的光电器件设计者来说，这可能是一个重要的发现，因为他们可以放心地在不显著降低性能的情况下引入少量的V缺陷。文章的最后部分标注了光学科学分类代码（OCIS codes: 160），这表明该研究主要集中在光学物理领域，特别是与材料的光学吸收特性和缺陷对光传输的影响有关的部分。这项第一原理的研究为深入理解和设计具有特定光学性质的CsI基半导体材料提供了理论依据。

74 CHINESE OPTICS LETTERS / Vol. 8, No. 1 / January 10, 2010

First-principles study on the optical properties

for CsI crystal with a pair of V

1−

-V

Tingyu Liu (444ÉÉÉ)

1,2∗

, Feinan Yan (îîîIII)

1,2

, Jun Chen ( ddd)

1,2

and Liping Liang (ùùùwww±±±)

1,2

College of Science, University of Shanghai for Science and Technology, Shanghai 200093, China

Shanghai Key Laboratory of Contemporary Optics System, Shanghai 200093, China

∗

E-mail: liutyyxj@163.com

Received March 16, 2009

The electronic structures and optical properties of both the perfect CsI crystal and the crystal containing

a pair of V

1−

-V

are calculated using CASTEP code with the lattice structure optimized. The calculated

results indicate that the optical symmetry of the CsI crystal coincides with the lattice structure geometry

of the CsI crystal. The absorption spectrum of the CsI crystal containing a pair of V

1−

-V

also does not

o ccur in the visible and near-ultraviolet range. It reveals that the existence of the pair of V

1−

-V

in CsI

crystal has no visible effects on the optical properties of the CsI crystal.

OCIS co des: 160.4670, 160.2220, 160.4760.

doi: 10.3788/COL20100801.0074.

Cesium iodide crystal is widely used as a scintillator

material in particle detectors due to its high emission

efficiency and fast decay time, especially the crystal

doped with thallium

[1−4]

. In general, this kind of sim-

ple cubic structured crystal would be easily damaged by

high-energy irradiation, such as X- or γ-ray, resulting in

the creation of color centers and making the crystal occur

absorption bands in the visible range.

The Kr ion irradiated CsI

[5]

exhibits several absorption

bands peaking at 240, 335, 410, 560, and 751 nm with

a shoulder at 716 nm. The absorption bands of color

centers are harmful to the scintillate properties. Many

researchers have discussed the origins of these absorp-

tion bands

[5−8]

. The CsI crystal after the irradiation

exhibits six absorption bands peaking at 375, 560, 750,

1020, 1180, and 285 nm

[9]

. The former five bands are

assigned to I

1−

color center, H band, F band, I

2−

color

center, and F

color center, respectively. The origin of

the absorption band at 285 nm is still unclear

[9]

. The

growth of optical absorption spectra in a high-Tl crystal

over a wide range of γ-ray dose show a group of six peaks,

which are at 355, 390, 430, 460, 520, and 560 nm. In

the infrared region, 840-nm absorption band originating

from the F-band and 980-nm absorption band related to

the F

-band are also observed

[10]

. The short wavelength

absorption bands are generally assigned to V band and

the long wavelength absorption bands are assigned to F

bands

[6]

On the other hand, there are a large number of suc-

cessful simulations on the optical prop erties of crystals

using the density functional theory (DFT)

[11,12]

with the

code called CASTEP. In order to further study the phys-

ical properties of the pair of V

1−

-V

related defects, the

electronic structures, dielectric function, complex refrac-

tive index, and absorption spectra of both the perfect CsI

cluster and the CsI cluster containing a pair of V

1−

-V

under the irradiation of a polarized light are calculated

using CASTEP code with the lattice structure optimized.

The lattice structure optimization is required before we

study the optical properties of the crystal due to the lat-

tice distortion caused by the existence of V

1−

-V

The CsI single crystal is simple cubic structured. The

CsI supercell used here consists of 27 Cs and 27 I centered

at I, having the chemical formula of [Cs

]. The CsI

supercell containing a pair of V

1−

-V

has a Cs vacancy

1−

nearest centered I ion and an iodine V

replacing

the centered I

1−

ions.

The CASTEP code is used for the lattice optimiza-

tion for the CsI containing one or two vacancies and the

calculation of the ground-state electronic structure us-

ing a plane-wave pseudo-potential formulation within the

framework of DFT with generalized gradient correction

in the form of PW91

[13−15]

. We use ultrasoft pseudo-

potentials for the iodine and cesium atoms and a plane-

wave cutoff energy of 340 eV. The basic parameters are

chosen as follows: kinetic energy cutoff of 340 eV, fast

Fourier transform (FFT) grid dimensions of 60×60×64,

space representation being reciprocal, and self-consistent

ﬁeld (SCF) tolerance 1.0×10

−6

of eV/atom. Optimal

atomic positions are determined until satisfying the con-

ditions: 1) the maximal force on them is smaller than

0.5 eV/nm; 2) the maximal change of energy per atom

is smaller than 0.00001 eV; and 3) the maximal displace-

ment is smaller than 0.0001 nm. All other calculations

are performed on basis of the lattice structure being op-

timized.

The dielectric function of an anisotropic material is a

complex symmetric second-order tensor which describes

the linear response of an electronic system to an applied

external electric field. The imaginary part of the dielec-

tric tensor is directly related to the electronic band struc-

ture of a solid, so it can be computed from the knowledge

of single-particle orbitals and energies approximated by

the solutions of the Kohn-Sham equations. However, it is

a well known fact that DFT calculations underestimate

the band gap. To take this into account, a ‘scissors oper-

ator’ is used, allowing a shift of the bands situated above

the valence band and a rescaling of the matrix elements.

1671-7694/2010/010074-04

° 2010 Chinese Optics Letters

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