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화학적변환에 의한 단결정 Te계 칼코지나이드 나노와이어의 합성 및 전기적 특성평가

Title
화학적변환에 의한 단결정 Te계 칼코지나이드 나노와이어의 합성 및 전기적 특성평가
Other Titles
Synthesis and Electrical Properties for Single-crystalline Te based Chalcogenide Nanowires by Chemical Transformations
Author
정다복
Alternative Author(s)
Jeong, Dabok
Advisor(s)
좌용호
Issue Date
2012-02
Publisher
한양대학교
Degree
Master
Abstract
칼코지나이드 (Chalcogenide) 소재는 주기율표에서 산소를 제외한 16족 원소인 칼코젠(Chalcogen) 원소 (S, Se, Te)를 하나이상 포함하는 이원계화합물로, 반도체이자 반금속으로 분류되는 소재이다. 칼코젠이나 칼코지나이드는 전자 및 광학소자에 적용될 수 있기 때문에 많은 연구자들의 관심을 받고 있다. 이 중에서 Se과 Te은 특이한 결정구조 및 여러 화학 반응물과의 반응이 쉽기 때문에 다른 기능성 물질로 화학적 변환이 가능하다. 기존 결정 물질에서 화학적 변환에 의해 다른 결정 구조의 물질로 변환되기 위해서는 원자 및 이온의 확산을 위한 높은 활성화에너지가 필요하다. 하지만 나노구조체에서는 부피에 따른 표면적의 증가로 상 변환 에너지가 현저하게 낮아지게 된다. 그로 인하여, 물질의 크기가 나노크기로 작아짐에 따라 물질의 화학적 변환이 쉽게 일어 날 수 있다. 본 연구에서는 3가지의 화학적 변환방법을 이용하여 Te나노와이어 및 Te계 금속칼코지나이드인 Ag2Te 및 CdTe 나노와이어를 합성하였다. 먼저, 갈바닉치환 반응을 이용하여 Si기판에 Te나노와이어를 수직으로 형성시켰으며, 형성된 나노와이어의 지름과 길이는 각각 87nm, 2.3µ
m이다. 갈바닉 치환반응에 의해 형성된 Te나노와이어의 형성 메커니즘에 대한 고찰을 위하여 반응용액의 조성에 따른 실험을 진행하였으며, Si기판의 영향을 알아보기 위하여 Si기판에 도핑된 불순물 농도 및 불순물 종류에 따른 실험을 진행하였고 각각의 형상 및 상을 분석하였다. 또한, Te나노와이어의 형성에서 온도의 영향을 알아보고자, 온도에 따른 형상 및 상을 분석하였다. 형성된 Te나노와이어의 온도에 따른 전기적 특성을 분석하였으며, 그에 따른 활성화에너지를 예측하였다. 갈바닉 치환반응에 의해 형성된 Te나노와이어를 topochemical 반응을 이용하여 Ag2Te나노와이어로 화학적 변환을 하였다. Hexagonal 구조를 갖는 Te격자 내에 Ag원자를 도입하여 monoclinic Ag2Te나노와이어를 형성하였으며, 반응 시간에 따른 상 변화 및 격자변화에 대해 관찰하였다. 이렇게 형성된 monoclinic구조를 갖는 Ag2Te나노와이어를 양이온치환 반응을 이용하여 zincblende 구조를 갖는 CdTe나노와이어로 변환시켰으며, 반응시간에 따른 상 변화를 관찰하였다. 이처럼 화학적 변환에 의해 형성된 나노와이어는 여러 번의 격자구조변환에도 초기에 형성된 형상을 그대로 유지하는 것을 확인하였다. 형상 변화 없이 상변화 및 격자변화를 이루었음을 본 연구에서 확인하였으며, 각각의 격자변화에 의한 밴드갭 변화 등에 의한 물성변화의 가능성을 확인하였다. 이렇게 형성된 각각의 나노와이어는 다양한 적용분야에 적용이 가능하다. Te은 0.35eV의 밴드갭에너지를 갖는 p타입 반도체로써, 반도체, 열전소자, 압전소자 등에 적용 될 수 있고, Ag2Te는 약 0.14eV의 밴드갭에너지를 갖는 물질로 자기저항소자 (Magnetoresistance)에 이용될 수 있으며, 1.44eV의 밴드갭을 가지는 CdTe는 고효율 태양전지로 이용이 가능하다.|Recently, chemical transformation of nanostructured materials has gained growing interest. Chemical transformation can be changed existing materials to various compositions of nanostructured materials. In addition, it might generate unexpected crystal structure due to the mechanical stress during the transformation. The transformation reactions are generally very slow because high activation energies are required for the diffusion of atoms and ions in the solid. In nanostructures, the thermodynamics and kinetics of reactions can change with size. As the solid size downs to nanoscale, the surface to volume ratio significantly increased, which lowers the phase transition energy. As a result, the chemical transformation easily occurs in nanostructured materials, especially one-dimensional (1-D) nanostructures. We have focused on the chalcogens and metal chalcogenide as an ideal system for the formation of 1-D nanostructures. Chalcogens are all group 16 elements of the periodic table (S, Se, Te) and chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element. For Te and Se atoms, the sum of bond energies for two single bonds is nearly double that of one double bond. As a result, Te and Se can form rings and long polymeric chains. The chain of these atoms can be readily crystallized into a hexagonal lattice through van der Waals interactions. Crystallization tends to occur along the c-axis induced by this structure, which favoring the stronger covalent bonds over the relatively weak, inter-chain van der Waals force. Therefore, these solid materials have a tendency to form highly anisotropic, 1-D structure. In this study, we focused on the synthesis of tellurium (Te) and metal telluride by chemical transformation using solution-based synthesis. We studied three kinds of chemical transformation, such as galvanic displacement reaction, topochemical reaction, cation exchange reaction. Through galvanic displacement reaction, we vertically synthesized Te nanowires on Si surface and converted into Ag2Te and CdTe by topochemical reaction and cation exchange reaction, respectively. We demonstrated the synthesis of Te nanowires on Si substrate in the solution containing Cd ions which have a role of inhibitor for preferential growth. We converted Te nanowires to Ag2Te nanowires intercalating Ag atoms into Te lattice during topochemical reaction. In addition, the synthesized Ag2Te nanowires were transformed into CdTe nanowires by cation exchange reaction. All of the synthesized nanowires maintained single-crystallinity during the chemical transformations, which were accompanied with phase changes. As the phase changes occurred on the nanowires due to the transformation, the band-gap energy also changed with the component of nanowires. Te has band gap energy of 0.35eV and can be applied on optoelectric, thermoelectric, piezoelectric materials. Ag2Te, band gap energy of 0.04~0.17eV, can be applied on thermoelectric device, semiconductor and magnetoresistances. CdTe with 1.45eV as band gap energy are applied to high efficiency solar cell. As the Te nanowires convert into Ag2Te, CdTe in this study, we can confirm the possibility to change of band gap energy and potential application.
Recently, chemical transformation of nanostructured materials has gained growing interest. Chemical transformation can be changed existing materials to various compositions of nanostructured materials. In addition, it might generate unexpected crystal structure due to the mechanical stress during the transformation. The transformation reactions are generally very slow because high activation energies are required for the diffusion of atoms and ions in the solid. In nanostructures, the thermodynamics and kinetics of reactions can change with size. As the solid size downs to nanoscale, the surface to volume ratio significantly increased, which lowers the phase transition energy. As a result, the chemical transformation easily occurs in nanostructured materials, especially one-dimensional (1-D) nanostructures. We have focused on the chalcogens and metal chalcogenide as an ideal system for the formation of 1-D nanostructures. Chalcogens are all group 16 elements of the periodic table (S, Se, Te) and chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element. For Te and Se atoms, the sum of bond energies for two single bonds is nearly double that of one double bond. As a result, Te and Se can form rings and long polymeric chains. The chain of these atoms can be readily crystallized into a hexagonal lattice through van der Waals interactions. Crystallization tends to occur along the c-axis induced by this structure, which favoring the stronger covalent bonds over the relatively weak, inter-chain van der Waals force. Therefore, these solid materials have a tendency to form highly anisotropic, 1-D structure. In this study, we focused on the synthesis of tellurium (Te) and metal telluride by chemical transformation using solution-based synthesis. We studied three kinds of chemical transformation, such as galvanic displacement reaction, topochemical reaction, cation exchange reaction. Through galvanic displacement reaction, we vertically synthesized Te nanowires on Si surface and converted into Ag2Te and CdTe by topochemical reaction and cation exchange reaction, respectively. We demonstrated the synthesis of Te nanowires on Si substrate in the solution containing Cd ions which have a role of inhibitor for preferential growth. We converted Te nanowires to Ag2Te nanowires intercalating Ag atoms into Te lattice during topochemical reaction. In addition, the synthesized Ag2Te nanowires were transformed into CdTe nanowires by cation exchange reaction. All of the synthesized nanowires maintained single-crystallinity during the chemical transformations, which were accompanied with phase changes. As the phase changes occurred on the nanowires due to the transformation, the band-gap energy also changed with the component of nanowires. Te has band gap energy of 0.35eV and can be applied on optoelectric, thermoelectric, piezoelectric materials. Ag2Te, band gap energy of 0.04~0.17eV, can be applied on thermoelectric device, semiconductor and magnetoresistances. CdTe with 1.45eV as band gap energy are applied to high efficiency solar cell. As the Te nanowires convert into Ag2Te, CdTe in this study, we can confirm the possibility to change of band gap energy and potential application.
URI
https://repository.hanyang.ac.kr/handle/20.500.11754/137346http://hanyang.dcollection.net/common/orgView/200000419604
Appears in Collections:
GRADUATE SCHOOL[S](대학원) > BIONANOTECHNOLOGY(바이오나노학과) > Theses (Master)
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