• Bulletin of the Korean Chemical Society
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• 제34권11호
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• pp.3335-3339
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• 2013

Boron-doped ZnO (BZO) nanorods were grown on quartz substrates using hydrothermal synthesis, and the temperature-dependence of their photoluminescence (PL) was measured in order to investigate the origins of their PL properties. In the UV range, near-band-edge emission (NBE) was observed from 3.1 to 3.4 eV; this was attributed to various transitions including recombination of free excitons and their longitudinal optical (LO) phonon replicas, and donor-acceptor pair (DAP) recombination, depending on the local lattice configuration and the presence of defects. At a temperature of 12 K, the NBE produces seven peaks at 3.386, 3.368, 3.337, 3.296, 3.258, 3.184, and 3.106 eV. These peaks are, respectively, assigned to free excitons (FX), neutral-donor bound excitons ($D^{\circ}X$), and the first LO phonon replicas of $D^{\circ}X$, DAP, DAP-1LO, DAP-2LO, and DAP-3LO. The peak position of the FX and DAP were also fitted to Varshni's empirical formula for the variation in the band gap energy with temperature. The activation energy of FX was about ~70 meV, while that of DAP was about ~38 meV. We also discuss the low temperature PL near 2.251 eV, related to structural defects.

• 한국전기전자재료학회논문지
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• 제24권12호
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• pp.992-995
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• 2011

The optical properties of ZnO thin film, being treated by O-plasma, have been studied using Photoluminescence(PL) spectroscopy with the change of temperature from 10 K to 290 K. Two characteristic peaks were identified at 10 K : 3.357 eV($D^{\circ}X$) and 3.324 eV(TES). The peak of $D^{\circ}X$ is believed to be due to neutral donor bound excitons where the donor is in the ground state. However, the TES(Two Electron Satellite) peak indicates the excited state of the donor(excitation energy was ~30 meV). The donor binding energy was estimated to be 44 meV, which indicates the possible presence of the neutral donor bound excitons at RT. The thermal effect including thermal broadening was identified from temperature evolution of the spectrum. Both the peak intensity and the peak energy have decreased as the temperature increases. As the temperature approaches to RT, the two peak merges into one broad peak, which is considered a combination of multiple peaks having different physical origins.

• 한국전기전자재료학회논문지
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• 제26권2호
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• pp.83-86
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• 2013

The optical properties of ZnO thin film have been studied using photoluminescence(PL) spectroscopy with the change of sample temperatures from 10 K to 290 K. The spectrum at 10 K showed the characteristic emission lines of ZnO which were as follows: free exciton(FX) at 3.369 eV, neutral donor-bound exciton($D^0X$) at 3.360 eV, two electron satellite(TES) at 3.332 eV, $D^0X$-1LO at 3.289 eV, and donor-acceptor pair(DAP) transiton at 3.217 eV. From the spectral evolution with temperatures, two features could be identified as temperature went higher: (1) the bound excitons changed gradually into free excitons, (2) DAP turned into free electron-acceptor transition(e,$A^0$). The PL intensity of free exciton increased with the increase of temperatures, which was accompanied by the decrease of the intensity of bound excitions and bound excition-related transitons such as TES and $D^0X$-1LO. At 80 K DAP transition disappeared, while (e,$A^0$) transition started to appear at 30 K.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.467.1-467.1
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• 2014

Quantum dots (QD) solar cells has received considerable attention due to their potential of improving the overall conversion efficiency by harvesting excess energy via multiple excitons generation (MEG). Although there have been many reports which show MEG phenomena by using optical measurement of quantum dots themselves, carrier multiplication in real QD photovoltaic devices has been sparsely reported due to difficulty in dissociation of excitons and charge collection. In this reports, heterojunction QD solar cells composed of PbS QD monolayer on highly crystalline $TiO_2$ thin films were fabricated by using Langmuir-Blodgett deposition technique to significantly reduce charge recombination at the interfaces between each QD. The PbS CQDs monolayer was characterized by using UV-vis, transmission electron microscopy (TEM) and atomic force microscopy (AFM). The internal quantum efficiency (IQE) for the monolayer QD solar cells was obtained by measurement of external quantum efficiency and determining light absorption efficiency of active layer. Carrier multiplication was observed by measuring IQE greater than 100% over threshold photon energy. Our findings demonstrate that monolayer QD solar cell structure is potentially capable of realizing highly efficient solar cells based on carrier multiplication.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2014년도 제46회 동계 정기학술대회 초록집
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• pp.103.1-103.1
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• 2014

Due to their excellent optical and electrochemical properties, conjugated polymers have attracted much attention over the last two decades and employed to opto-electrical devices. In particular, conjugated polymers possess many attractive features that make them suitable for a variety of sensing task. For example, their delocalized electronic structures can be strongly modified by varying the surrounding environment, which significantly affected molecular energy level. In other word, conjugated polymers can detect and transduce the environmental information into a fluorescence signal. Conjugated polymers also display amplified quenching compared to small molecule counterparts. This amplified fluorescence quenching is attributed to the delocalization and migration of the excitons along the conjugated polymer backbones. Long backbones of conjugated polymer provide the transporting path for electron as a conduit, allowing that excitons migrate rapidly into quencher site along the backbone. This is often referred to as the molecular wire effect or antenna effect. Moreover, structures of conjugated polymers can be easily tailored to adjust solubility, absorption/emission properties, and regulation of electron/energy transfer. Based on this versatility, conjugated polymers have been utilized to many novel sensory platforms as a promising material. In this tutorial, I will highlight a variety of fluorescence sensors base on conjugated polymer and explain their sensory mechanism together with selected examples from reference literatures.

• Electronic Materials Letters
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• 제14권6호
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• pp.766-773
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• 2018

In this paper, temperature dependence of the excitonic bands in a mechanically exfoliated tungsten diselenide ($WSe_2$) monolayer is studied using photoluminescence and circular dichroic photoluminescence (PL) in the temperature range between 8 and 300 K. The peak energies associated with the neutral exciton (A), charged exciton (trion) and localized excitons are extracted from the PL spectra revealing a trion binding energy of around 30 meV. The circular dichroic PL measured at 8 K shows about 45% valley polarisation that sharply reduces with increasing temperature to 5% at 300 K with photoexcitation energy of 1.96 eV. A detailed analysis of the emission line-width suggests that the rapid decrease of valley polarisation with the increase of temperature is caused by the strong exciton-phonon interactions which efficiently scatter the excitons into different excitonic states that are easily accessible due to the supply of excess photoexcitation energy. The emission line-width broadening with the increase of temperature indicate residual exciton dephasing lifetime < 100 fs, that correlates with the observed rapid valley depolarisation.

• 한국결정성장학회지
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• 제8권2호
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• pp.211-220
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• 1998

수평 전기로에서 $AgGaS_2$ 다결정을 합성하여 HWE 방법으로 $AgGaS_2$ 단결정 박막을 성장하였다. $AgGaS_2$ 단결정 박막을 성잘할 때 증발원과 기판의 온도를 각각 $590^{\circ}C$, $440^{\circ}C$로 성장하였을 때 이중결정 X-선 요동곡선(double crystal X-ray diffraction rocking curve, DCRC)의 반폭치(FWHM)값이 124 arcsec로 가장 작아 최적 성장조건이었다. 상온에서 $AgGaS_2$ 단결정 박막의 광흡수 특성으로부터 에너지 띠간격이 2.61cV였다. Band edge에 해당하는 광전도도 peak의 온도 의존성은 Varshni 관계식으로 설명되었으며, Vaeshni 관계식의 상수값은 Eg(0) = 2.7284eV, $\alpha$= 8.695$\times$10-4 eV/K, $\beta$= 332K 로 주어졌다. 광발광 봉우리는 20K에서 414.3nm(2.9926eV)와 414.1nm(2.7249eV)는 free exciton(Ex)의 upper polariton과 lower polariton인 {{{{{E}`_{x} ^{u} }}}}와 {{{{{E}`_{x} ^{L} }}}}, 423.6nm(2.9269eV)는 bound exciton emission에 의한 I로 관측되었다. 또한 455nm(2.7249eV)의 peak는 donor-acceptor pair(DAP)에 기인하는 광발광 봉우리로 관측되었다.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2002년도 제4회 영호남학술대회 논문집
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• pp.84-91
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• 2002

The stochiometric composition of $AgGaS_2$/GaAs polycrystal source materials for the $AgGaS_2$/GaAs epilayer was prepared from horizontal furnace. From the extrapolation method of X-ray diffraction patterns it was found that the polycrystal $AgGaS_2$/GaAs has tetragonal structure of which lattice constant an and Co were 5.756 $\AA$ and 10.305 $\AA$, respectively. $AgGaS_2$/GaAs epilayer was deposited on throughly etched GaAs(100) substrate from mixed crystal $AgGaS_2$/GaAs by the Hot Wall Epitaxy (HWE) system. The source and substrate temperature were $590^{\circ}C$ and $440^{\circ}C$ respectively. The crystallinity of the grown $AgGaS_2$/GaAs epilayer was investigated by the DCRC (double crystal X-ray diffraction rocking curve). The optical energy gaps were found to be 2.61 eV for $AgGaS_2$/GaAs epilayer at room temperature. The temperature dependence of the photocurrent peak energy is well explained by the Varshni equation, then the constants in the Varshni equation are given by $\alpha=8.695{\times}10^{-4}$ eV/K, and $\beta=332K$. From the photocurrent spectra by illumination of polarized light of the $AgGaS_2$/GaAs epilayer, we have found that crystal field splitting ${\Delta}Cr$ was 0.28 eV at 20 K. From the PL spectra at 20 K, the peaks corresponding to free and bound excitons and a broad emission band due to D-A pairs are identified. The binding energy of the free excitons are determined to be 0.2676 eV and 0.2430 eV and the dissociation energy of the bound excitons to be 0.4695 eV.

• 한국정보디스플레이학회:학술대회논문집
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• 한국정보디스플레이학회 2006년도 6th International Meeting on Information Display
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• pp.571-576
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• 2006

We review work performed by our group and collaborators in the area of exciton formation. There are three components: (i) measurement of singlet fractions, (ii) analysis of magnetic resonance measurements, and (iii) models of exciton formation. We find that the fraction of singlet excitons is a controllable quantity, pointing the way to a new generation of all-fluorescent organic light emitting devices.

• 한국재료학회:학술대회논문집
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• 한국재료학회 1998년도 IUMRS-ICEM ABSTRACT BOOK
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• pp.71.3-71
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• 1998