Conjugated molecules are receiving much attention as promising materials with high charge-carrier mobility and environmental stability for molecule-based electronics.[1] These include not only organic thin-film devices (i.e., organic light emitting diodes (OLED), organic thin-film transistors (OTFT), and organic photovoltaic (OPV)) as well as molecular devices based on a single molecule. Using π-conjugated molecules in electronic devices is also expected to offer excellent controllability in geometric and electronic structures of active components which play a critical role in determining device functionality.[2]

It is now recognized that interfacial contacts between π-conjugated molecules and metal electrodes play roles as important as the molecules themselves in molecule-based electronics. As shown in Figure 1, the interfaces between molecules and metal electrodes are where charge transfers between active components and electrodes occur in organic thin-film devices, and such interfacial contacts are also present in any persuasive model for molecular devices.[2] In addition, various interactions at the interfaces between molecules and metal electrodes (i.e., surface-molecule and intermolecular interactions) are of great importance in organic epitaxy and the architecture of nanostructures or molecular networks in reference to size and structural control of the system,[3-5] and the registry of a molecule on a metal surface governed by interfacial interactions has strong correlation with the subsequent film growth and its electronic structure.[6, 7] Thus, how to understand and control interfacial interactions is one of today's challenging issues not only in the field of surface science, but also in various other fields of science and engineering.

Throughout my doctoral studies, I investigated "how to understand and control various interfacial interactions between π-conjugated molecules and noble metal surfaces" with well-designed model systems by means of scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. Noble metals have been widely used as electrodes for molecule-based devices owing to their chemical inertness, differently from other transition metals which cause oxidation or instability.[1, 8] The use of noble metal substrates makes it possible to control surface-molecule interactions corresponding to the work-function of metal, surface lattices and surface templating. Moreover, since π-conjugated molecules normally lie flat on noble metal surfaces, tuning interfacial interactions can be achieved by careful design of the molecules. Here, the geometric and electronic structures of π-conjugated molecules on noble metal surfaces were probed by STM and STS measurements, and DFT calculations also performed to elucidate the experimental results and to describe interfacial interactions in detail.

The Most Intrinsic Interfacial Interactions in Weak Adsorption System of a π-Conjugated Molecule on the Noblest Metal Surface

A well-controlled model system of a π-conjugated molecule on the noblest Au(111) A well-controlled model system of a π-conjugated molecule on the noblest Au(111) surface was studied to investigate the most intrinsic interfacial interactions between a π-conjugated molecule and the noblest metal surface. Frontier orbitals of molecular adsorbates play a crucial role in the adsorption process when it involves chemical interactions via orbital hybridization. However, in the case of "weak" non-bonding adsorption (so-called physisorption) of a π-conjugated molecule on the noblest Au surface, it is controversial as to whether orbital interaction still contributes to determining adsorption geometry and structure. Most difficulties involved in probing this issue arise mainly from the chemical inertness of the Au surface[8] when added to the practical problems in the assignment of exact adsorption geometries of weakly adsorbed species at the single-molecule level caused by a lower activation barrier for diffusion. Thus far van der Waals (vdW) interaction has been considered the principal way of describing such a weak adsorption system. The possibility of orbital interaction has been excluded thus far due to such difficulties, and vdW interaction with a shallow potential minimum has been the main consideration in describing the adsorption geometry and structure of a π-conjugated molecule on the Au surface. To disentangle the difficulties in studying such weak adsorption, a well-defined system of dehydrobenzo[12]annulene (DBA) on the Au(111) surface was used. DBA has a well-matched three-fold symmetry with the Au(111) surface as well as a planar π-conjugated framework without any functional groups that might chemically interact with the surface Au atoms.

Because low coverage deposition at room temperature prevents lateral interaction between adsorbates and enables reliable detection of the proper geometry of DBA at the single-molecule level, and since the Au atoms in the vicinity of DBA may be observed by varying the tunneling conditions and DBA appears as an easily distinguishable triangular shape, it is feasible to accurately determine the adsorption geometry of DBA/Au(111), which adsorbs to a hollow site as shown in Figure 2. Based on the experimentally determined adsorption structure, DFT calculations were performed to elucidate interplay between DBA and the Au(111) surface in weak adsorption. By comparison of inequivalent local minimum structures, it has been clearly shown that structural stability of the system originates mainly from the electronic structures at the molecule-surface interface. Energy alignment between the π state of DBA and the Au 5d_(z^2 ) states gives rise to a difference in the interaction strength corresponding to molecular orientation, which is related to the degree of overlap between the molecular orbital states of DBA and the Au d states. The variation in the strength of orbital interaction corresponding to the adsorption geometries is well explained by a qualitative comparison of the partial charge density (PCD) distributions as shown in Figure 3, and the optimal interfacial distance is also closely related to the energy level alignment between the π components of DBA and the Au d states. Further experiments to probe the orbital interaction of a π-conjugated molecule on the noblest Au surface showed that it can play a critical role even in elementary surface processes (i.e., lateral manipulation of a single molecule, and further two-dimensional (2D) molecular assembly).

Control of Various Interactions at the Interface between π-Conjugated Molecules and Noble Metal Surfaces

Molecular ordering and the structure of π-conjugated molecules on a noble metal surface are normally governed by a complex process that operates through various surface-molecule and intermolecular interactions (e.g., covalent bonding, hydrogen bonding, dipole-dipole interaction, metal-ligand coordination, vdW interaction, and surface templating) at the interface between molecules and surfaces.[4, 5] The numerous factors involved in the prediction and control of molecular structures make systematic study and sophisticated molecule design essential to the achievement of desired structures. The control of variables for this research topic is mainly achieved by careful selection of the metal substrate and chemical tuning of the molecule.

First, to exclude the surface templating effect in the molecular ordering of DBA on noble metal surfaces, the Ag(111) surface was used for comparison with the Au(111) surface. As there is no surface templating effect on Ag(111), the molecular ordering of the DBA molecules is widely uniform on the Ag(111) surface as opposed to the chain-like structure of DBA along the face-centered-cubic (fcc) domain of Au(111). The intrinsic interfacial interaction between the molecule and the surface (i.e., the interaction between the π components of the molecule and the d states of the noble metal) is still maintained even though the strength of the interaction locally differs due to a change in the metal work-function. In addition, the transition of molecular ordering corresponding to coverage of the molecule was easily achieved on the Ag(111) surface, because intermolecular interaction is more strongly reflected on surfaces not involved in surface templating.

Second, controllable molecular ordering on the Au(111) surface has been demonstrated through the chemical tuning (alkoxy substitution) of DBA. Fine-tuning the subtle balance between molecule-molecule and surface-molecule interactions by changing the primary intermolecular interaction through modification of the alkoxy group promises to permit control of the formation of molecular ordering. The π-conjugated triangular core of each alkoxylated DBA derivative is locked on a specific site on the Au(111) surface by the interaction between -O- and the Au surface atoms. Under this precondition, the intermolecular interactions have been tuned from the hydrogen bonding interaction to the chain-chain vdW interaction by substituting alkoxy groups (see Figure 4). Such tunable intermolecular interactions enable control of molecular ordering, i.e., restricting the number of polymorphs, and the introduction of domain-specific chirality.

Finally, more complex system was studied using bis([1,2,5]thiadiazolo)-tetracyanoquinodimethane (BTDA-TCNQ). By comparison of molecular ordering on various noble metal surfaces (i.e., Au(111), Ag(111), and Cu(111)), different growth mechanisms were investigated aimed at achieving the formation of widely uniform geometric and electronic structures. In addition to STM measurements, STS and STS mapping techniques were used to elucidate the electronic structures in combination with DFT calculations. Although data analysis is still ongoing, it is expected that the study will demonstrate anisotropic electron confinement owing to strong intermolecular interaction well-balanced with surface-molecule interaction.[9] For potential applications, molecular structures on amorphous Au and the vicinal Au(788) surfaces have been demonstrated on the basis of the result on Au(111).

In summary, my doctoral studies are aimed at proposing a better way to understand interfacial contacts between π-conjugated molecules and noble metal surfaces, and methodology to control interfacial interactions. Since interfacial interactions have strong correlation with geometric and electronic structures of the system as well as single adsorption events, I anticipate that this study will provide not only deeper insight into the interfacial contacts between π-conjugated molecules and noble metal surfaces, but also a general basis for designing the architectures of molecule-based electronics.

Publication

4. Ju-Hyung Kim, Jaehoon Jung, Miyabi Imai, Eisuke Ohta, Yuki Suna, Takanori Fukushima, Takuzo Aida, Yousoo Kim and Maki Kawai. Geometric and electronic structures of organic-metal hybrid system: bis([1,2,5]thiadiazolo)-tetracyanoquinodimethane on noble metal surfaces. in preparation (2012).

3. Juyeon Park, Ju-Hyung Kim, Jaehoon Jung, Kazukuni Tahara, Yoshito Tobe, Yousoo Kim and Maki Kawai. Steering organic crystalline thin film via cooperative interfacial and π-conjugated molecular interaction. in preparation (2012).

2. Ju-Hyung Kim, Jaehoon Jung, Kazukuni Tahara, Yoshito Tobe, Yousoo Kim and Maki Kawai. Orbital interaction in weak adsorption of a π-conjugated molecule on the noblest gold surface. in preparation (2012).

1. Ju-Hyung Kim, Kazukuni Tahara, Jaehoon Jung, Steven De Feyter, Yoshito Tobe, Yousoo Kim and Maki Kawai. Ordering of molecules with π-conjugated triangular core by switching hydrogen bonding and van der Waals interactions. submitted (2012).

Presentation

2. "How does a pi-conjugated molecule recognize a specific site on the noblest metal surface?" (Oral), Japan Physical Society (JPS) Fall Meeting, Japan, September 2011.

1. "Two dimensional molecular networks of triangular dehydrobenzo[12]annulene and its derivatives on the Au(111) surface" (Oral), Asian Conference on Nanoscience and Nanotechnology 2010 (AsiaNANO2010), Japan, November 2010.

Figure 1. Schematic illustration of charge-injection in organic thin-film devices.

Figure 2. Experimentally determined adsorption geometry of DBA/Au(111). Top and bottom regions were obtained with a sample bias (Vs) of -2 mV and tunneling current (It) of 9.0 nA, whereas the middle region was obtained at Vs of -500 mV and It of 0.4 nA.

Figure 3. PCD distributions for two local minimum states. The experimentally determined adsorption structure (designated "α") is most favorable, followed by a second local minimum structure (designated "β"). Dashed line indicates a center of the first layer of Au(111), and the positions of six carbon atoms of DBA are depicted by dots.

Figure 4. STM images of 2D molecular network formed by alkoxylated derivatives of DBA (Vs = 2.0 V, It = 0.2 nA).

本論文は、走査トンネル顕微鏡(Scanning Tunneling Microscope, STM)、走査トンネル分光(Scanning Tunneling Spectroscopy, STS)と密度汎関数理論(Density Functional Theory, DFT)計算を效果的に用いることで明らかになった、π電子系分子と不活性金属の界面における相互作用を体系的に論じた研究成果について、報告している。論文は英文で 7章からなり、第1章は研究背景及び本研究の理解に必要な概念の導入, 第2章は STM/STS 及び DFTの解説, 第3章以降は結果及び考察である。第3章は化学的に最も不活性な金属表面と知られているAu(111)表面と無極性の π電子系分子(Dehydrobenzo[12]annulene, DBA)の間の弱い相互作用に関する実験及び理論的研究、第4章は異なる金属表面による效果とそれによる DBA 分子の超薄膜形成過程を Ag(111) 表面の上で実験的に観察した研究、 第5章は DBA 分子の化学構造を調節して、分子間の相互作用制御を利用した単分子薄膜構造の形成に対する実験及び理論的研究、第6章は分子間に強い相互作用がある分子(Bis([1,2,5]thiadiazolo-tetracyanoquinodimethane, BTDA-TCNQ)を利用した不活性なAu(111)表面の広域に形成する分子薄膜の均一な幾何学的構造及び電子構造の研究、そして第7章は結語である。以下、章ごとにその内容を詳しく述べる。

第1章では、まず本論文の目的を要約し記述している。本論文の主要な研究内容は不活性金属とπ電子系分子の吸着に関与する相互作用の研究で、金属と分子間の相互作用と吸着した分子の間の相互作用を理解し制御することである。また不活性金属表面と吸着分子の相互作用に関する先行研究等を紹介し、界面に存在する相互作用の理論的モデルを簡単に紹介している。中でも本論文で主に扱っているπ電子系分子の吸着については特に詳しく記述されている。次に、不活性金属とπ電子系分子界面の相互作用は幾何学的構造及び電子構造を左右する因子であること、さらに分子間の相互作用にも影響を与える因子であることを指摘している。またそれらに関する先行研究が紹介されている。第1章の終わりに本論文の主要な結果の要約を記述している。

第2章では, 本研究で使用したSTMとSTS、STSマッピングの手法に関する基本原理と実際の研究に使用した実験装置について詳しく記述している。次に、実験結果を理論的に説明するために用いたDFT計算の原理と計算方法について詳しく記述している。

第3章では、不活性な金属表面とπ電子系分子の間に存在する基本的相互作用に関する研究として、最も不活性なAu(111)表面とDBA 単分子の間の弱い吸着に関連する相互作用をSTM/STS 及びDFTを用い解析した研究の結果を記述している。これまで明らかではなかった弱い吸着系での電子軌道間の相互作用及び部分的な電子移動が行われることを明らかにし、このような相互作用が分子の吸着構造を決定づける重要な役割を果たすことを論じている。一般的に、不活性なAu(111)表面の上に弱く吸着する分子の吸着構造を明らかにする事は難しいが、分子の三回対称構造の特徴を生かし、低温STMの実験技術を駆使し単分子レベルの吸着構造を明らかにした。STMから得られた単分子の吸着構造をSTS 及びDFT 計算によって詳細に考察し、不活性な金属とπ電子系分子の弱い相互作用が構造決定に重要な鍵となることを解き明かした。次に、このような単分子レベルの吸着構造が薄膜構造形成にも影響を与え、分子の非対称的な配列を誘導することができるということもSTM実験結果で明確に示した。

第4章では、第3章で使用したπ電子系分子である DBAを異なる仕事関数を持った不活性金属であるAg(111)表面に吸着させて、比較考察した結果について記述している。STM/STS実験からAu(111)表面に比べて強い相互作用を有するということを明らかにした。さらに、Au(111)表面とは異なりAg(111) 表面は表面再構成をしないため、分子間の相互作用と分子薄膜の形成及び分子の配列過程がより明確に現れた。

第5章では、DBA 分子の誘導体を用いてAu(111)表面上で分子間の相互作用及び分子配列をSTM及びDFTにより研究した結果を記述している。DBA 分子に異なる長さのアルコキシ基を付加した実験では、分子間の相互作用を水素結合からファンデルワールス力に切り替えることができることを実験と計算結果で示し、それにより分子集合体の構造がAu(111)表面の上で変化することを見出した。このような分子間の相互作用の制御は、分子薄膜を構成する分子の配列や均一性を效果的に変え得る可能性を示唆しており、分子薄膜に分子の非対称的な配列を誘導することが可能であることもSTM 実験から示している。

第6章では、Au(111)表面の広い領域で均一な幾何学的構造及び電子構造を持つ分子薄膜形成を STM/STS 及び STS マッピングを用いて観察した結果を記述している。構成材料には、分子間に大きな電荷移動が知られているBTDA-TCNQを用いた。これら分子はAu(111)との相互作用は相対的に弱いので、BTDA-TCNQ分子間の強い相互作用が十分に作用し広い面積で均一な分子薄膜を形成することを見いだした。またSTSマッピングの手法を效果的に利用してAu(111)表面とBTDA-TCNQ分子薄膜の界面における電子状態も詳細に記述した。電子状態も幾何学的構造と同じく非常に均一な構造をもつことが分かり、分子の電子状態に反映される界面の電子状態の選択性を論じている。さらに、応用分野への展開を視野に、Au(788)および非結晶金表面の上で分子の薄膜形成に関する研究結果についても記述している。

第7章は結語であり、本博士論文で解明された成果を簡潔にまとめている。

なお論文提出者は、本論文の全章の研究に関して実験及び計算部分に寄与した。また第5章は共同研究であるが、論文提出者が主体的に実験及び理論的解析を行った。従って、論文提出者の寄与が十分であると判断し、博士(科学)の学位を授与できると認める。