Two-dimensional Interlocking Chiral Honeycomb Structures of an Unnatural Amino Acid in a Single Domain
We have studied the two-dimensional self-assembled structures of a functionalized unnatural amino acid, $(S)-\beta-methyl$ $naphthalen-1- \gamma$-aminobutyric$ acid $(\gamma^2 -1-naphthylalanine)$, to investigate the formation of self-assembled nanostructures at 150 K by using scanning tunneling microscopy (STM), density functional theory (DFT) calculations, and high resolution photoemission spectroscopy (HRPES). Interestingly, STM analysis at a low coverage revealed interlocking chiral honeycomb structures that consist of both counter-clockwise and clockwise configurations in a single domain, which suggests that the intermolecular interac-tions between $\gamma^2$ -1-naphthylalanine molecules play a crucial role in the formation of those self-assembled structures in spite of the low coverage. As the amount of adsorbed $\gamma^2$ -1-naphthylalanine was increased, a well-ordered square closed packing structure was observed along with another structural domain in which higher molecules are trapped in the pores of the hexagonal molecular assembly. On the other hand, naphthalene on a Au(111) surface only forms a well-ordered hexagonal closed packing structure at high coverages. HRPES analysis indicates that there is no strong covalent bonding between $\gamma^2$ -1-naphthylalanine and the Au(111) surface, supporting that $\gamma^2$ -1-naphthylalanines have strong tendency to form the self-assembled structures mainly through intermolecular interactions. Thus, we conclude that modifying molecules to facilitate intermolecular interactions can potentiate the formation of well-ordered self-assembly nanostructures.
Comparison of the Coverage-dependent Adsorption Selectivity of Leucine and Tyrosine adsorbed on the Cu(110) Surface
The coverage-dependent adsorption selectivity of leucine and tyrosine molecules adsorbed on the Cu(110) surface was investigated and compared to obtain the bonding configurations and stable adsorption structures at two distinct coverages (low and high). These features were determined using density functional theory (DFT) calculations, high-resolution photoemission spectroscopy (HRPES), and reflection-absorption infrared spectroscopy (RAIRS). The DFT calculations indicates that an “O-H dissociation bonded structure” is the most stable structure of leucine on the Cu(110) surface, and an “O-H dissociated and N-dative bonded structure” is the most stable structure of tyrosine on the Cu(110) surface. In addition, according to HRPES and RAIRS results of at low coverage level (~0.20 ML), leucine shows “O-H dissociation bonded structure” on the Cu(110) surface whereas tyrosine shows “O-H dissociated and N-dative bonded structure” on the Cu(110) surface. In contrast, at higher coverage (~0.70 ML), we found the opposite results that the leucine on the Cu(110) surface shows “O-H dissociated and N-dative bonded structure” whereas tyrosine on the Cu(110) surface shows “O-H dissociation bonded structure”. These results were consistent with the results of the DFT calculations, which indicate that adsorption geometries yield the lowest adsorption energies at each coverage.
Nitrogen Doping via $NH_3$ Treatment as a Strategy for Increasing $Pd-TiO_2$ Surface Defect Sites Related to Catalytic Activity
This study examined the catalytic activities of three distinct nitrogen doped $Pd@TiO_2$ ($N-Pd@TiO_2$) nanoparticles, post-annealed $(at 700, 800, and 900^\circ C)$ after fabrication on silicon substrates. The catalytic activities were analysed by measuring the oxidation of benzenethiol, thioacetic acid, and thiobenzoic acid by using X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), scanning transmission x-ray microscopy (STXM), high-resolution photoemission spectroscopy (HRPES), and scanning photoelectron mi-croscopy (SPEM). Systematically, the study by HRPES and SPEM indicated that nitrogen was predominantly doped around PdO nanostructure. Particularly, we prove that photocatalytic activity in effective nitrogen doped surface area is far higher than in undoped area of $Pd@TiO_2$ nanoparticles. Besides to the band gap narrowing, nitrogen doping leads to generate $Ti^{3+}$ species or oxygen vacancies site of the surface, which leads to enhance the activity of surface photocatalysis.
비천연 아미노산의 맞물린 벌집 키랄 구조체 연구
Au(111) 표면 위에서 비천연 아미노산인, $(S)- \beta-methyl naphthalen-1- \gamma -aminobutyric$ acid $(\gamma^2 -1-naphthylalanine)$ 의 이차원 자기 조립 구조체를 150 K에서 주사 터널링 현미경 (Scanning Tunneling Microscopy, STM)과 고분해능 광전자 분광법 (High-Resolution Photoemission Spectroscopy, HRPES)을 이용하여 연구하였다. 흥미롭게도, 낮은 coverage에서 시계 방향과 반시계 방향의 벌집 키랄 구조체가 하나의 domain에 함께 나타난다는 것을 주사 터널링 현미경을 이용하여 확인하였다. $\gamma^2$ -1-naphthylalanine의 기능화기 사이의 수소결합이 낮은 coverage임에도 자기조립 구조체를 형성할 수 있도록 중요한 역할을 했다고 여겨진다. $\gamma^2$ -1-naphthylalanine의 coverage를 증가시켰을 때, 잘 정렬된 사각 조밀 쌓음 구조와 육각 분자 조립체의 구멍 안에 잡힌 분자 구조를 형성한 다는 것을 확인하였다. 이와 반대로, Au(111) 표면 위에서 나프탈렌은 높은 coverage에서만 정렬된 육각 조밀 쌓음 구조를 형성하였다. 또한, 고분해능 광전자 분광법을 통해 $\gamma^2$ -1-naphthylalanine과 Au(111) 사이의 강한 결합이 없다는 것을 확인하였고, 이는 $\gamma^2$ -1-naphthylalanine분자들 사이의 상호작용이 자기 조립 구조체를 형성하는 주된 요인이라는 것을 뒷받침해준다.
Cu(110)에서 leucine과 tyrosine의 흡착 구조 비교 연구
Cu(110) 표면 위에서 leucine과 tyrosine의 흡착 구조를 낮은 coverage와 높은 coverage에서 밀도 범함수 이론 (Density Funcional Theory, DFT) 계산과 고분해능 광전자 분광법 (High-Resolution Photoemission Spectroscopy, HRPES), 초고진공 퓨리에변환 적외선분광기 (Reflection-Absorption Infrared Spectroscopy, RAIRS)를 이용하여 연구하였다. 밀도 범함수 이론 계산을 통해 leucine의 흡착 구조 중 “O-H dissociation bonded structure”가 가장 안정한 구조이고, tyrosine의 흡착 구조 중 “O-H dissociated and N-dative bonded structure”가 가장 안정한 구조라는 것을 밝히었다. 또한, 고분해능 광전자 분광법과 적외선 분광법을 통해 낮은 coverage에서, leucine은 “O-H dissociation bonded structure”, tyrosine은 “O-H dissociated and N-dative bonded structure”라는 것을 규명하였다. 반대로, 높은 coverage에서는, leucine은 “O-H dissociated and N-dative bonded structure”, tyrosine은 “O-H dissociation bonded structure”를 갖는 다는 것을 확인하였다. 이 결과는 밀도 범함수 이론계산의 결과와도 잘 맞는다는 것을 알 수 있었다.
촉매활성에 관련된 $PdTiO_2$ 표면의 결함을 증가시키는 질소 도핑
$Pd@TiO_2$ 에 질소를 도핑한 후 700, 800, 900도 씨에서 열처리를한 세 종류의 나노입자의 촉매 활성을 연구하였다. 고분해능 광전자 분광법 (High-Resolution Photoemission Spectroscopy, HRPES)과 주사 광전자 현미경(Scanning photoelectron microscopy, SPEM)을 이용하여 질소가 PdO의 주위에 탁월하게 도핑되는 것을 확인하였다. 특히, 벤젠싸이올, 싸이오아세트산, 싸이오벤조산의 산화에 대한 촉매 활성을 측정하여, 질소가 도핑된 $Pd@TiO_2$ 표면이 도핑이 되지 않은 $Pd@TiO_2$ 표면보다 훨씬 큰 광촉매 활성을 보이는 것을 확인하였다. 이러한 결과들은 질소 도핑이 밴드갭을 줄여주는 것뿐만 아니라, $Ti^{3+}$ 나 산소 빈자리를 만들어서 표면의 광촉매 반응을 효과적으로 향상시키는 것을 보여준다.
