Assignment of NEXAFS Resonances in Alkanethiols and Their Implication on the Determination of Molecular Orientation of Aliphatic SAMs.
Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy has become a widely used technique to analyze the molecular orientation of functional adlayers by analyzing the dichroism of NEXAFS resonances. In contrast to π-conjugated systems, which exhibit distinct π-resonances whose transition dipole moments are oriented perpendicular to the aromatic planes, the analysis of purely aliphatic systems is not as straightforward and requires a precise identification of the underlying resonances and their polarization. Here we analyze the carbon K-edge NEXAFS signature and its dichroism of a self-assembling monolayer (SAM) formed by octadecanethiol (ODT) on gold representing the widely studied class of alkanethiol-SAMs. Employing density functional theory calculations and using the transition potential method, the polarizations of all NEXAFS resonances at excitation energies around and below the ionization threshold are precisely determined. This information is then used to simulate expected dichroism curves for various adlayer arrangements which are compared with our experimental findings for ODT-SAMs on single crystalline Au(111) and polycrystalline gold surfaces. This analysis demonstrates a novel strategy to precisely deduce the molecular orientation from the experimental NEXAFS data of aliphatic SAMs.
dichroism of NEXAFS resonances : 여러 Polarized 된 빛을 이용해서 분석한다는 거겠지?
aliphatic – non aromatic
NEXAFS spectroscopy는 표면에 붙은 adlayer를 (chemically absorbed) NEXAFS resonance는 dichroism을 통해 분석하는데 많이 이용된다. SAM octadecanethiol (ODT)-on gold 의 carbon K-edge NEXAFS를 분석한다. 왜냐하면 n-conjugated system이 n-resonances (transision dipole moments are oriended perpendicular to the aromatic planes) 를 가지는 거랑 다르게 aliphatic 분자는 그 resonance가 잘 알려지지 않았어. DFT 랑 transition potential method를 이용하니까, polarization of all NEXAFS resonances 가 ionization threshold 를 결정지었어. 이걸 가지고 여러 arrangements에 대한 expected dichroism curve를 simulate했어. 그리고 그 결과를 실험 결과와 비교했지.
Q; n-conjugate system 이랑 비교해서 NEXAFS 로 aliphatic의 arrignment를 보는게 왜 힘든지 나는 잘 모르겠다. -> 공부해야 할 것이군
structure and thermal stability of stilbenedithiol SAMs on Au(111).
Self-assembling monolayers (SAMs) of dithiols play an important role as divalent linker layer in applications such as photoelectrochemical sensors. However, their formation processes and film structure are not fully understood yet. Herein, we investigate a SAM of aromatic transstilbenedithiol (StDT) on Au(111) surfaces. Using complementary characterization techniques we investigated the dependence of the film quality on preparation parameters, such as used solvent, temperature and ambient conditions during and subsequent to preparation. Furthermore, we found a remarkably high thermal stability which we ascribe to disulfide bridging between the endgroups of the individual molecules.
divalent : 2가
divalent ion : 2가 이온
Dithiol의 divalent linker layer은 photoelectrochemical sensor에서 중요한 열학을 하는데, 그 과정을 잘 몰라. 그래서 SAM of aromatic transstilbenedithiol (StDT) on Au(111)를 연구할 건데, 여기에서는 preparation 과정 중 또는 후에 solvent, 온도 주변 조건들을 바꿔가면서, 그게 quality에 어떻게 영향을 미치는지 알아볼거야. 게다가 Thermal stability가 높은걸 발견했어.
Copper Phthalocyanine as Contact Layers for Pentacene Films Grown on Coinage Metals.
The metal-semiconductor interface determines the efficiency of charge carrier injection into any organic electronics device. Control of this interface, its structure and morphology is therefore essential for device improvement. In this study, we analyze the approach of controlling semiconductor morphology at this interface by insertion of a copper phthalocyanine (CuPc) monolayer as a primer between Ag(111), Au(111) and Cu(100) surfaces and the organic semiconductor pentacene (PEN). Controlled monolayer formation is facilitated by thermal desorption of excess multilayers, monitored via thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), and the growth of PEN on the resultant monolayer primers is investigated by near-edge X-ray absorption spectroscopy (NEXAFS), atomic force microscopy (AFM) and STM. While well-ordered CuPc monolayers with flat-lying molecules are formed on Au(111) and Ag(111), no long-range order is observed on Cu(100). Subsequently deposited PEN molecules initially adopt a recumbent orientation with their long axis oriented parallel to the surface, while upon further deposition this structure is metastable as molecules adopt an upright orientation beyond the bilayer and form (001) oriented films. Although the recumbent orientation of the CuPc primer layer is not transferred to thicker PEN films, which is attributed to the geometrical inequality of the two molecules, a distinct dewetting, as found for PEN films grown on bare metal surfaces, is efficiently suppressed. This effect is reproducible even for polycrystalline Au surfaces, which resemble the situation of metal contacts in devices.
charge carrier가 들어가는 금속/반도체 경계는 Organic electronics device에서 그 효율성을 결정짓는다. 그래서 이 경계면의 구조와 morphology의 통제는 장치의 능력을 향상시키는데 가장 중요한 부분이다. 여기에서 우리는 CuPc를 monolayer를 primer로 Ag(111), Au(111) and Cu(100) surfaces 와 the organic semiconductor pentacene (PEN) 사이에 넣어서 morphology를 통제하는 방법에 대해서 연구했다. Thermal desorption of excess multilayers로 (multilayer를 만들어 놓은 상태에서 가열해가지고 monolayer만 남게 함) 통제된 monolayer 를 만들었고, 그 과정은 TDS, XPS, STM로 지켜보았다. Prmer 위에 PEN를 자란 것은 NEXAFS, AFM, STM으로 조사했다. 잘 정렬된 CuPc monolayers 가 flat-lying방향으로 Au(111) and Ag(111) 위에 만들어진 반면에, Cu(100) 에서는 딱히 큰 범위를 아우르는 정렬을 형성하지 못했음을 (no long-range order) 관찰했다. 다음으로 deposited PEN 는 우선은 누운 구조를 취했고 반면에 deposition을 더 하니까 bilayer 이후의 구조는 upright (001) 방향을 취했다. primer layer인 누은 구조의 CuPc가 두꺼워지는 PEN film을 따라서 변하지는 않았지만, PEN의 distinct dewetting (맨 금속일 경우 나타나는)은 효율적으로 억제되었다. 이 현상은 장치의 금속 접촉 상황과 유사한 polycrystalline Au 표면에서도 발견되었다.
Low-lying excited states in crystalline perylene
Organic materials are promising candidates for advanced optoelectronics and are used in light-emitting diodes and photovoltaics. However, the underlying mechanisms allowing the formation of excited states responsible for device functionality, such as exciton generation and charge separation, are insufficiently understood. This is partly due to the wide range of existing crystalline polymorphs depending on sample preparation conditions. Here, we determine the linear optical response of thin-film single-crystal perylene samples of distinct polymorphs in transmission and reflection geometries. The sample quality allows for unprecedented high-resolution spectroscopy, which offers an ideal opportunity for judicious comparison between theory and experiment. Excellent agreement with first-principles calculations for the absorption based on the GW plus Bethe–Salpeter equation (GW-BSE) approach of many-body perturbation theory (MBPT) is obtained, from which a clear picture of the low-lying excitations in perylene emerges, including evidence of an exciton–polariton stopband, as well as an assessment of the commonly used Tamm–Dancoff approximation to the GW-BSE approach. Our findings on this well-controlled system can guide understanding and development of advanced molecular solids and functionalization for applications.
Au, Ag 랑 Cu 사이에 있는 PEN semiconductor에 CuPc를 넣어서 semiconductor morphology를 통제하는 방법을 연구했다. Controlled monolayer는 TDS, XPS, STM 을 통해서 연구했고, the resultant monolayer primers 위에 PEN 결정이 형성된 것은 NEXAFS, AFM STM. CuPc monolayer가 Au(111), Ag(111) 위에 잘 형성된데 반해서, Cu(111) 위에서 그러지는 못했어. 다음으로 PEN deposition은 누워있는 CuPc의 구조를 따라서 긴 axis가 표면에 평행하게 셩성되었지. 근데 더 deposition 하고 보니까, 누운 구조는 metastable이었어. bilayer이후 부터는 (001) 방향으로 섰지. CuPc의 누운 layer는 PEN을 따라서 서지 않았어. 그래도 a distinct dewetting on bare metal surface는 약해졌어. (억제되었어). Au polycrystallin e 표면에서도 이 현상이 나타났지.
Crystalline Packing in Pentacene-like Organic Semiconductors.
Since optoelectronic properties of organic semicond uctors (OSCs) are largely affected by the molecular packing in the solid phase, further advances of such materials require c omprehensive structure-property interrelations beyo nd single molecule considerations. While single molecular ele ctronic properties can be tailored by synthetic mea ns and their electronic properties can be reliably predicted by quantum chemical calculations, crystal structure pr edictions of such van der Waals bond solids remain challenging. Here we a nalyze correlations between the molecular structure and the resulting packing motifs adopted in the crystalline phases of the prototypical OSC pentacene as well as various differently substituted but similarly shaped π -conjugated molecules. Based on a Hirshfeld surface analysis and related fingerprint plots, specific contact points and their distributi on are identified which allows a classification of different structure groups. Comparing the fingerprint plots with corresponding molecular properties such as electrostatic contour plots as well as quadrupole and polarizability tensors, which were c alculated by density functional theory, allows rati onalizing structure determining specific intermolecular interactions. O ur analysis shows in particular that molecules with uniform electrostatic potential at their periphery favor a herringbone pa cking, while the highly electronegative substituent s (O, N and F) enable the formation of H-bonds and prefer slip-stacking o r criss-cross packing motifs. The present correlati ons might be useful guidelines for future strategies to synthesis new O SCs.
prototypical OSC pentacene as well as various differently substitutedd but similarly shaped π -conjugated molecules 로 분자 구조랑 그 구조에 따른 결정 구조가 어떻게 결정되는지에 대해서 볼거야. Hirshfeld surface analysis와 related fingerprint plot 을 기본으로 어느 부분으로 접촉하는지, 어떤 distribution을 띄는로 structure group을 나눠봤어. 각각의 분자 특성에 따른 fingerprint plot를(예를 들어 electrostatic contour plots, quadropol and polrizability tensors by DFT) 비교해서 분자구조가 특정한 intermolecular inteaction을 결정 짓는 합리적인 연결 고리를 만들어냈어? allows rati onalizing structure determining specific intermolecular interactions (대충 뜻은 이해했는데 한국말로 적절하게 안 바꿔지네 ) 주변부에 균일한 electrostatic potential이 있는 분자들을 herringbon paking을 선호하고 반면에 electronegativie substituents가 주변부에 있는 분자는 H-bonds를 만들수 있고 slip-staking or criss-crossing morifs하는 경향을 보였어.