In this thesis, emission control of phosphors and quantum dots (QDs) is reported to achieve excellent white light sources and new yellow-emitting phosphors for white light-emitting diodes (LEDs) are introduced. And the combination of phosphor with semiconductor QDs is firstly tried for the generation of white light. In addition, this thesis shows that quantum calculation using density functional theory (DFT) is useful for the analyses of electronic structure of phosphors and the design of new phosphors. And it is shown that yellow-emitting phosphors in this study can be applied to white light sources using various primary sources [e.g., carbon nanotubes (CNTs)] as well as white light sources using LEDs.
In chap. 3, widely used yellow-emitting $Y_3Al_5O_{12}:Ce^{3+}$ (YAG:Ce)-based phosphors are discussed. Yellow-emitting $Y_{2.94-x}Al_5O_{12}:Ce^{3+}_{0.06}$, $Pr^{3+}_x$ (YAG:Ce,Pr) phosphors where x = 0, 0.006, 0.015, 0.03, and 0.06 were synthesized to enhance red spectral intensity and YAG:Ce,Pr is reported in section 3.1. Sharp red emission peaking at about 610 nm was observed from $Y_3Al_5O_{12}:Ce^{3+}$, $Pr^{3+}$ (YAG:Ce,Pr) and it is ascribed to the transition of $^1D_2$ → $^3H_4$ of $Pr^{3+}$. Although red emission peak was observed in YAG:Ce,Pr, yellow emission intensity decreased as the amount of $Pr^{3+}$ was increased. To investigate the origin of the decrease of yellow emission via the electronic transition of $5d^1$ → 4f of $Ce^{3+}$, crystal structure, particle size, and energy transfer were considered. It is suggested that the decrease of yellow emission is attributed to the radiative or non-radiative energy transfer from $Ce^{3+}$ to $Pr^{3+}$. In the case of non-radiative energy transfer, energy transfer occurred from $Ce^{3+}$ to $Pr^{3+}$ via dipole-dipole interaction and calculated critical distance for the energy transfer was 12$\textrm{\AA}$.
In Section 3.2, $Tb^{3+}$ -substituted YAG:Ce [(Y, Tb)AG:Ce] is dealt with. Yellow-emitting (Y, Tb)AG:Ce phosphors where x = 0, 0.02, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0 were synthesized. As the amount of $Tb^{3+}$ was increased in the YAG:Ce phosphor, $Ce^{3+}$ emission band shifted to a longer wavelength due to larger crystal field splitting induced lower site symmetry and shorter Ce-O distance. At the same time, $Ce^{3+}$ emission decreased with increasing $Tb^{3+}$. Various factors which would have influence on PL intensity of YAG:Ce phosphor were investigated. (Y, Tb)AG:Ce phosphor showed pure garnet structure and no agglomeration. From the decay time measurement, life time of 5d → 4f transition of $Ce^{3+}$ decreased from 64.4 to 21.7 ns as the amount of $Tb^{3+}$ was increased, that indicates that energy transfer from $Ce^{3+}$ to $Tb^{3+}$ occurred. In the consequence, it was believed that the decrease of emission intensity of (Y, Tb)AG:Ce was attributed to the $Ce^{3+}$ → $Tb^{3+}$ energy transfer.
When YAG:Ce was co-doped with Pr or substituted by Tb, color coordinates of the phosphor was tuned but yellow emission intensity was decreased. Therefore, other yellow-emitting phosphors are discussed in chap. 4. In section 4.1, $Eu^{2+}$ -activated $Sr_3 SiO_5$ was investigated as a phosphor for the application to white light sources using blue/near ultraviolet (n-UV) LEDs. $Sr_3SiO_5 :Eu^{2+}$ showed a bright orange-yellow emission under blue or n-UV light excitation. As $Eu^{2+}$ concentration increased in the $Sr_3SiO_5$ host lattice, it was observed that emission wavelength shifted to longer wavelengths (redshift). To verify conspicuous variation of $Eu^{2+}$ emission with change of crystal field splitting, effect of substitution of $Ca^{2+}$ and $Ba^{2+}$ ions into $Sr^{2+}$ sites was investigated. When Sr sites were substituted by Ba, emission band of $(Sr, Ba)_3SiO_5 :Eu^{2+}$ shifted to a longer wavelength (Ba < 20 mol%) and then shifted to a shorter wavelength (Ba < 20 mol%) due to the competition of crystal field effect and Nephelauxetic effect. It was also investigated color coordinates and color rendering property of the blue LED-pumped white light source with ${(Sr_{1-x} M_x)}_3 SiO_5 :Eu^{2+}$. The n-UV LED-pumped white light source was also fabricated and luminescent property of it was investigated.
In section 4.2, I have evaluated luminescent properties of a $Ba_3SiO_5:Eu^{2+}$ phosphor. When $Ba_3SiO_5:Eu^{2+}$ was excited by blue light of 450 nm, it showed one emission band (581 nm), while it showed photoluminescence (PL) spectrum consisting of two subbands at 504 nm and 581 nm under excitations of 405 nm or 370 nm. Due to different crystal field splitting of $Eu^{2+}$ at the two different Ba sites, $Eu^{2+}$ ions showed orange-yellow emission (581 nm) and green emission (504 nm) depending on their occupying sites, respectively. Critical distance for energy transfer was considered using spectral overlap and concentration quenching and calculated critical distances were 17.1 $\angst$ and 18.5 $angst$, respectively. When an InGaN-based blue LED was coated with the mixture of $Ba_2SiO_4:Eu^{2+}$ and $Ba_3SiO_5:Eu^{2+}$, it showed a bright daylight-like white light (luminous efficiency of 40.0 lm/W, color coordinates (x, y) = (0.3160, 0.2950), color temperature of 6584 K, and high color rendering index of 85).
Chapter 5 is concerned with new yellow-emitting $Ce^{3+}$ -activated silicate phosphor and the white LEDs fabricated by using the phosphors. Since both $Sr_3 SiO_5:Eu^{2+}$ and $Ba_3SiO_5:Eu^{2+}$ phosphors need additional green-emitting phosphor to obtain high color rendering property of the white LEDs, new yellow-emitting phosphors are necessary and new composition of $Sr_3SiO_5 :Ce^{3+} ,Li^+$ is introduced in chap. 5. In section 5.1, a yellow-emitting $Sr_3SiO_5 :Ce^{3+}$, $Li^+$ phosphor is reported. Through transitions of 5d → 4f ($^2F_{7/2}$ and $^2F_{5/2}$) in $Ce^{3+}$, the phosphor showed a very broad and strong yellow emission under n-UV or blue light excitation. The energy levels of $Ce^{3+}$ in $Sr_3SiO_5$ were suggested from its absorption and PL excitation (PLE) spectra. The critical distance for the energy transfer between $Ce^{3+}$ ions was calculated to be about 17 $\angst$ by considering spectral overlap and concentration quenching. White light could be obtained by combining this phosphor with 460 nm or 405 nm LEDs [(x, y) = (0.3086, 0.3167) or (0.3173, 0.3103)]. Additionally, a yellow LED was fabricated using a n-UV LED (380 nm chip) with $Sr_3SiO_5:Ce^{3+},Li^+$.
In section 5.2, optical properties of white light-emitting devices fabricated by using $Sr_3SiO_5:Ce^{3+}$, $Li^+$ phosphors are evaluated. White LEDs were fabricated by combining a yellow-emitting $Sr_3SiO_5:Ce^{3+}$, $Li^+$ phosphor with a blue LED (460 nm chip) or an n-UV LED (405 nm chip), respectively. Color temperature ( $T_c$ ) of $Sr_3SiO_5:Ce^{3+}$, $Li^+$ -based white LEDs could be tuned from 6,500 K to 100,000 K (blue LED pumping) and from 4,900 K to 50,000 K (n-UV LED pumping) without mixing with other phosphors. The blue LED-pumped white LED showed excellent white light (luminous efficiency = 31.7 lm/W, Tc = 6857 K) at 20 mA. This white LED showed stable color coordinates property against an increase of the forward bias current. An n-UV LED-pumped white LED also showed bright white light (25.0 lm/W, 5784 K) at 20 mA.
In Chapter 6, it is reported that synthesized yellow-emitting phosphors were applied to blue LEDs and optical properties of the white LEDs using various yellow-emitting phosphors were investigated. In section 6.1, optical properties of white LEDs fabricated by combining InGaN-based blue LEDs with highly luminescent $Tb_3 Al_5 O_{12}:Ce^{3+}$ (TAG:Ce), $Y_3Al_5O_{12}:Ce^{3+}$ (YAG:Ce), and $Sr_3SiO_5:Eu^{2+}$ (SS:Eu) are described. The TAG:Ce-based white LED showed a color rendering index ( $R_a$ ) of 79, and luminous efficiency ( $η_L$ ) of 34.1 lm/W at 20 mA. The YAG:Ce-based white LED and the SS:Eu-based white LED showed low Ra values of 75 and 57 but high luminous efficiency values of 38.9 lm/W and 41.3 lm/W at 20 mA, respectively. When a mixture of YAG:Ce and SS:Eu was coated on a blue LED and the resultant white LED operated at 20 mA, the white LED showed a highly bright white light similar to daylight ( $η_L$ = 40.9 lm/W, color temperature $T_c$ = 5716 K, and $R_a$ = 76). Moreover, the white LED showed stable color coordinates against a considerable variation of applied current.
In section 6.2, the effect of the coated phosphor on the optical property of phosphor converted white LEDs is reported. A yellow-emitting TAG:Ce phosphors was coated onto blue LEDs to obtain white LEDs. Since TAG:Ce showed 90% of the brightness of YAG:Ce, it was expected that TAG:Ce-based white LEDs showed 90% of brightness of YAG:Ce-based ones. However, the TAG:Ce-based white LED showed 74% of the brightness of YAG:Ce-based one. Considering both density and mean size of the phosphors, higher density and larger size of TAG:Ce $(6.085 g/cm^3, 21.4 μm)$ than YAG:Ce $(4.657 g/cm^3, 16.6 μm)$ induced a great deal of sedimentation of TAG:Ce particles in an epoxy resin. It is believed that it is one of main reasons for the reduced optical power of the TAG:Ce-based white LED compared to that of the white LED expected from the brightness of TAG:Ce.
From the evaluation of optical properties of yellow-emitting phosphor-converted (YPC) white LEDs in chap. 6, there is a limitation in the color rendering property of YPC white LEDs. Therefore, in this thesis additional red-emitting component was considered. Chapter 7 deals with semiconductor quantum dots (QDs) as a candidate to improve color rendering property of YPC white LEDs. In section 7.1, luminescent properties of CdSe QDs are reported. CdSe QDs were synthesized by modified organometallic method. Emission band of CdSe QDs could be tuned from blue to red light by controlling their size (ca. 2.7 to 5.1 nm). This emission color variation is attributed to quantum confinement effect. Synthesized CdSe QDs showed wurtzite structure and very narrow size distribution (standard deviation of the size distribution < 5%). Although Quantum yield (QY) of core CdSe was below 40%, CdSe/ZnSe core/shell QDs showed higher QY of 79% thanks to the surface passivation.
In section 7.2, CdSe QDs-assisted $Sr_3SiO_5:Ce^{3+}$, $Li^+$ -based white LEDs are discussed. High quality red-emitting CdSe QDs were applied to LEDs. An average size of red-emitting CdSe QDs with high monodispersity (standard deviation ~ 3%) was about 4.8 nm. Due to the high monodispersity CdSe QDs showed 2 dimensional superlattice. Red LEDs and purplish-pink LEDs were fabricated by combining blue LED with red-emitting CdSe QDs. A white LED fabricated with a blue LED and the combination of greenish yellow-emitting $Sr_3SiO_5:Ce^{3+}$, $Li^+$ and red-emitting CdSe QDs showed natural white [(x, y) = (0.2904, 0.2900)] and excellent color rendering property $(R_a = 90)$. In addition, the white LED showed stable color coordinates property against the increase of forward bias current from 20 to 70 mA.
Although a new yellow-emitting phosphor was introduced in chap. 5 of this thesis, a certain systematic approach is necessary to develop additionally new yellow-emitting phosphors for white light generation. In chap. 8, quantum calculation (or first principle calculation) using density functional theory (DFT) is introduced to the phosphor systems. In section 8.1, the electronic structure of $Ba_3 SiO_5 :Eu^{2+}$ was obtained by using CAmbridge Serial Total Energy Package (CATEP) code. When $Eu^{2+}$ ions occupy different Ba sites, total energies of $Ba_3 SiO_5 :Eu^{2+}_{Ba(1)}$ and $Ba_3 SiO_5 :Eu^{2+}_{Ba(2)}$ were calculated to be -12616.30367 and -16426.38099 eV, respectively. The energy gap between 5d and 4f of $Eu^{2+}$ at Ba(2) sites is smaller than that of $Eu^{2+}$ ions at Ba(1) sites. These calculation results were consistent with observed luminescent results of $Ba_3 SiO_5 :Eu^{2+}$ in which emission band depends on the excitation wavelength.
I could confirm that quantum calculation using DFT can be applied to analysis of electronic structure of a phosphor system. In section 8.2, quantum calculation using DFT was employed to search new yellow phosphors for white LEDs. Electronic structures of $Ca_2 SiO_4$ and $Ca_2 SiO_4 :Ce^{3+} ,Li^+$ were obtained by using Vienna Ab-initio Simulation Package (VASP) code. $Ce^{3+}$ doping into $Ca_2 SiO_4$ host generated additional energy band between conduction band and valence band. The difference between the lowest 5d band and 4f level of $Ce^{3+}$ was very small and it implies that this new phosphor system can be applied to white LED using blue LEDs. $Ca_2 SiO_4 :Ce^{3+} ,Li^+$ was synthesized by solid state reaction method and the phosphor showed PLE band in n-UV to blue spectral region and broad PL band peaking at about 560 nm. Very short decay time of about 70 ns supports that the yellow emission is attributed to the transition of 5d → 4f in $Ce^{3+}$ ions. White LEDs were fabricated by combining a new yellow-emitting $Ca_2 SiO_4 :Ce^{3+} ,Li^+$ phosphor with a blue LED (455 nm chip) or an n-UV LED (405 nm chip), respectively. And their optical properties were characterized.
Additionally, chap. 9 concerns with a planar white light source using carbon nanotubes (CNTs). A strong yellow emission of YAG:Ce mixed with ZnS:Ag,Cl under electron excitation is reported in section 9.1, The penetration depths of electron of 1 keV and photon of 2.7 eV in YAG:Ce were estimated to be approximately 1450 $\angst$ and 4.65 mm, respectively. Deeper penetration of blue light from ZnS:Ag,Cl helps to excite a larger number of $Ce^{3+}$ in a mixture (ZnS:Ag,Cl + YAG:Ce), and YAG:Ce showed strong yellow emission via both cathodoluminescence (CL) and PL. The mixture showed the brightness of 120.5% compared to R, G, B phosphor mixture. This mixture of two phosphors was applied to a carbon nanotube field emission backlight unit.
TAG:Ce and SS:Eu were used to generate various white lights and two-phosphor systems are described in section 9.2. CL properties of blend of TAG:Ce and ZnS:Ag,Cl and blend of SS:Eu and ZnS:Ag,Cl were investigated. Although TAG:Ce and SS:Eu showed weak yellow emission under irradiation of electron beam, respectively, both TAG:Ce mixed with ZnS:Ag,Cl and SS:Eu mixed with ZnS:Ag,Cl showed stronger yellow emission ascribed to secondary blue light excitation as well as primary electron beam irradiation. This is due to longer penetration depth of blue light (4.58 and 4.07 mm for TAG:Ce and SS:Eu) than that of electrons (463 and 572 $\angst$ for TAG:Ce and SS:Eu). Bright white light which have different color coordinates could be generated from the blend of ZnS:Ag,Cl and yellow-emitting phosphors (TAG:Ce or SS:Eu) under the irradiation of electron beam.
액정 디스플레이 장치 (LCD - liquid crystal display)의 후면 광원 (Backlight unit)으로 사용되고 있는 냉음극형광램프 (CCFL - cold cathode fluorescent lamp)나 기존의 형광등 및 백열등과 같은 광원을 대체하기 위해서는 백색광의 구현이 필수적이다. 최근 기존 백색광원의 한계를 뛰어넘는 백색 발광 다이오드 (LED - light-emitting diodes)와 탄소 나노 튜브 (CNT - carbon nanotube)를 이용한 백색 면광원이 주목을 받고 있다. 이중 백색 LED는 형광등과 달리 Hg가 들어있지 않아 친환경적 일뿐만 아니라, 수명이 길고, 광변환 효율이 높아 에너지 소모가 작다. 이와 같은 장점들로 인해 LED는 차세대 광원으로 가장 각광 받고 있는 광원이다. 청색 LED 위에 황색 발광 $Y_3Al_5O_{12}:Ce^{3+}$ (YAG:Ce) 형광체를 도포하여 백색광을 구현하는 경우 고휘도의 백색 LED를 용이하게 제조할 수 있어 핸드폰 액정의 backlight units, 휴대폰의 카메라 플래쉬, 키패드 광원 등에 널리 이용되고 있다. 그러나 YAG:Ce 형광체는 적색 스펙트럼 영역의 발광이 강하지 않아 YAG:Ce을 이용한 백색 LED의 경우 연색 특성 - 광원이 물체의 원래 색을 재현하는 정도를 나타내는 특성 - 이 좋지 않은 단점이 있다. 따라서 LED 제조 회사 및 여러 연구 그룹에서는 백색 LED의 연색 특성을 향상시킬 수 있는 형광물질에 대하여 연구를 진행하고 있다.
본 연구에서는 황색 발광 형광체의 발광 스펙트럼을 조절하여 적색 스펙트럼 영역의 발광 강도를 증가시키고자 하였다. 먼저 YAG:Ce 형광체를 기반으로 하여 Pr을 첨가함으로써 약 610 nm의 적색 스펙트럼 영역에 발광 peak이 생성됨을 확인하였다. 이러한 $Pr^{3+}$ 이온의 발광 peak은 $Ce^{3+}$ 이온의 잔광 시간 측정 결과 $Ce^{3+}$ 이온으로부터의 발광-비발광 천이에 의한 것으로 확인되었다. 또한 YAG:Ce의 Y 자리에 Tb을 치환하는 경우 결정장의 세기가 증가하여 $Ce^{3+}$ 이온의 발광 밴드가 장파장쪽으로 이동하였다. 그러나 이 경우 역시 치환되는 Tb양이 증가 함에 따라 $Ce^{3+}$ → $Tb^{3+}$ 에너지 전달을 통해 황색광의 발광 강도가 감소하였다.
YAG:Ce의 적색 스펙트럼 영역의 발광 강도를 증가시키기 위해 Pr을 첨가하거나 Tb을 치환하는 경우, 황색광의 발광이 감소하였다. 따라서 non-YAG:Ce 계열인 실리케이트계 황색 형광체를 합성하였고 그 광특성을 분석하였다. $Sr_3SiO_5:Eu^{2+}$ 형광체의 경우 강한 오렌지 발광을 나타내었고, 단파장쪽으로 발광밴드를 이동시키기 위하여 Sr 자리를 Ba로 치환하였다. 이때 Sr과 Ba의 이온 반경과 전기음성도 차이 때문에 나타나는 결정장 효과와 Nephelauxetic 효과의 상호 작용으로 인해 발광 밴드가 장파장으로 이동한 후 단파장으로 이동하는 현상이 관찰되었다. Ba이 Sr을 100% 치환한 $Ba_3SiO_5:Eu^{2+}$ 형광체의 경우, $Eu^{2+}$ 이온이 Ba(1) 자리와 Ba(2)자리에 들어감에 따라 각각 녹색과 오렌지색 발광을 나타내었고, 이는 각 자리의 symmetry와 $Eu^{2+}-O^{2-}$ 의 거리가 다르기 때문으로 설명될 수 있다. 이러한 가설은 CASTEP (Cambridge Serial Total Energy Package) 프로그램을 이용하여 양자 역학 계산을 통해 Schrödinger 방정식을 풀어 전자 밀도 함수를 구함으로써 확인되었다. $Eu^{2+}$ 이온을 활성제로 하는 $Sr_3SiO_5:Eu^{2+}$, $Ba_3SiO_5:Eu^{2+}$ 형광체는 오렌지 발광을 나타내어 백색 LED 제조 시 연색 특성의 개선이 크게 이루어지지 않았다.
따라서 본 연구에서는 발광 폭이 넓은 황색광을 나타내는 $Ce^{3+}$ 이온을 활성 이온으로 하는 실리케이트 형광체를 개발하였다. 이제까지 $Ce^{3+}$ 이온이 산화물계 모체에서 황색광을 발광하는 경우는, garnet 계열 형광체 뿐이었으나 본 연구를 통하여 신조성 산화물계 황색 발광 $Sr_3SiO_5:Ce^{3+},Li^+$ 형광체가 보고되었다. $Sr^{2+}$ 이온 자리에 $Ce^{3+}$ 이온이 치환되므로 전하를 맞춰주기 위해 $Li^+$ 이온을 함께 도핑 하였다. $Sr_3SiO_5:Ce^{3+}$, $Li^+$ 형광체는 짧은 잔광 시간, 넓은 발광 폭, $Ce^{3+}$ 4f 준위의 spin-orbit coupling 등의 발광 특성을 보였으며, 이로부터 $Ce^{3+}$ 이온이 황색 발광을 나타냄을 확인하였다. 또한 $Ce^{3+}$ 이온의 5d 밴드의 결정장 분리가 크기 때문에 greenish-yellow부터 yellow light을 발광하는 특성을 나타내었다. $Sr_3SiO_5:Ce^{3+}$, $Li^+$ 형광체를 청색 LED에 도포한 경우 81이라는 높은 연색지수를 나타내었으며, 이는 $Sr_3SiO_5:Ce^{3+}$, $Li^+$ 형광체가 밴드 폭이 넓은 황색광을 나타내기 때문이다.
본 연구를 통하여 합성된 YAG:Ce 계열 및 $Eu^{2+}$ 이온을 활성이온으로 한 실리케이트 형광체, 그리고 신조성 황색 발광 $Sr_3SiO_5:Ce^{3+},Li^+$ 형광체를 각각 청색 LED에 도포하여 백색 LED를 제조하였고, 색좌표, 발광 효율, 연색 특성, 전류 안정성 등을 측정하였다. 그 결과 $Sr_3SiO_5:Ce^{3+}$, $Li^+$ 형광체를 이용하여 제조된 백색 LED의 경우, 형광체의 넓은 발광 밴드로 인해 높은 연색지수를 나타내었을 뿐 아니라 (Ra = 81), 30.1 lm/W의 높은 발광 효율을 나타내었고 인가 전류 증가에 대해서도 안정적인 색좌표 특성을 나타내었다.
그러나 85 이상의 고연색지수를 가지는 백색 LED를 구현하기 위하여는 청색 LED와 황색 발광 형광체뿐 아니라 적색 발광 물질이 추가로 필요하다. 이에 본 연구에서는 CdSe 양자점을 합성하였고 새로운 적색 발광 물질로 적용하여 LED로의 적용 가능성을 제시하였다. CdSe 양자점의 크기를 약 2에서 5 nm 까지 변화시켜 청색에서 적색까지 발광색을 조절할 수 있었으며, 약 5 nm 크기의 CdSe는 적색 발광을 나타내었다. 본 연구에서는 균일한 크기의 높은 결정성을 가지는 CdSe가 합성되었으며, 합성된 CdSe 양자점은 규칙적인 2차원 초격자 구조를 나타내었고 약 36.5%의 양자 효율을 보였다. 본 연구를 통하여 개발된 신조성 황색 발광 $Sr_3SiO_5:Ce^{3+}$, $Li^+$ 형광체와 CdSe 양자점을 동시에 청색 LED에 도포하여 백색 LED를 제조하였을 때 약 90의 고연색지수를 얻을 수 있었다. 그러나 발광 효율이 15 lm/W이하의 낮은 값을 나타내었으며, 이를 개선하기 위해 CdSe/ZnSe core/shell구조의 양자점을 $Sr_3SiO_5:Ce^{3+}$, $Li^+$ 형광체와 함께 이용하여 백색 LED를 제조한 결과 약 25 lm/W의 발광 효율을 나타내었다. 이것은 ZnSe shell이 CdSe의 surface passivation 역할을 하여 양자점의 발광 효율이 크게 향상되었기 때문이다.
새로운 형광체를 합성하는데 있어 본 연구뿐 아니라 대부분의 경우 educational guess 방법이 사용된다. 그러나 보다 체계적으로 신조성 형광체를 개발하기 위하여 본 연구에서는 제1원리 계산을 도입하였다. 먼저, Density functional theory를 이용한 양자계산을 통하여 $Ba_3SiO_5:Eu^{2+}$ 의 에너지 구조 최적화를 수행하고 에너지 밴드 구조, 전자 밀도 분포 등을 구하였다. 이렇게 이론적으로 구해진 값들을 이용하여 실험 결과를 잘 설명할 수 있었으며, 이를 통하여 제1원리 계산을 신조성 형광체 탐색에 적용할 수 있음을 확인하였다. 이를 바탕으로 density functional theory를 이용한 양자계산을 통해 이제까지 발광 특성이 보고된 바 없는 $Ca_2SiO_4:Ce^{3+}$, $Li^+$ 형광체에 대하여 구조 최적화 수행 후, 에너지 준위 및 density of states를 구해 황색 발광의 가능성을 확인하였다. 양자계산의 수행결과, 황색 발광의 가능성을 보인 $Ca_2SiO_4:Ce^{3+}$, $Li^+$ 형광체를 합성하였고, 합성된 형광체가 황색 발광을 나타냄을 확인함으로써, 이론 계산을 통해 신조성 형광체를 예측할 수 있음을 확인하였다.
LED를 이용한 백색광 구현 이외에도 앞서 합성된 황색 발광 형광체를 청색 발광 ZnS:Ag,Cl 형광체와 조합하여 CNT에 적용함으로써 백색 면광원을 구현하였다. 이 경우 CNT로부터 방출된 전자가 청색 발광 형광체와 황색 발광 형광체를 여기시키고 이와 동시에 청색 발광 형광체로부터 발생된 청색광이 황색 발광 형광체를 여기시키기 때문에 고휘도 백색광의 구현이 가능하였다.
본 연구를 통하여 신조성 무기 형광체가 보고되었으며, CdSe계열 양자점이 합성 되었다. 또한 형광체와 양자점의 발광 파장 변화를 통하여 색좌표 조절이 가능하였으며, 제1원리 계산을 통해 형광체의 에너지 준위가 분석되었다. 또한 무기 형광체와 양자점의 조합이 최초로 시도되었고, 우수한 광특성을 나타내는 백색 LED의 성공적인 제조로부터 그 응용 가능성이 확인 되었다. 이러한 결과는 앞으로 보다 우수한 백색 LED의 개발을 가능하게 할 것으로 예상되며, 형광체 및 양자점이 다방면으로 응용될 수 있는 방안을 제시할 것으로 생각된다.
