EXPERIMENTAL CHAPTER EXPERIMENTAL 2.1 Scanning Tunneling Microscopy (STM) There are many modern instruments for surface structural and chemical analysis such as the Field Ion Microscope (FIM), the Field Emission Microscope (FEM), Low-Energy Electron Diffraction (LEED), Scanning Electron Microscope (SEM), Electron Spectroscopy for Chemical Analysis (ESCA), Transmission Electron Microscope (TEM), etc. The development of these techniques has played an important role in the field of surface science. However, each of these techniques has certain strengths and limitations. LEED and X-ray diffraction techniques rely on large-scale order of the surface, and can at best give averaged information about local and defect structures; SEM requires samples with strong corrugation or mass contrast and its resolution is not high enough to resolve surface atoms. High-resolution TEM can in some cases resolve features with atomic dimensions of specially thinned samples. However this can be accomplished only by aligning the electron beam with the rows of atoms in a crystalline lattice. FEM and FIM are only able to probe the two-dimensional geometry of the atomic structure on the surfaces of sharp tips with radii less than 100nm. In addition, sample preparation is rather complicated. For FIM the samples must be stable in high fields, thus limiting its general usefulness. Other surface analytical techniques, such as X-ray Photoemission Spectroscopy (XPS), Ultraviolet Photoemission Spectroscopy (UPS) and Electron Energy Loss 15 EXPERIMENTAL Spectroscopy (EELS), can only provide spatially averaged information of the electronic structures of the surfaces. Moreover, some of the techniques mentioned above require high-vacuum environment and can only provide indirect results or strongly rely upon model systems for data interpretation. Until the Scanning Tunneling Microscope (STM) was introduced, it still remained a dream to directly observe geometric and electronic surface structures at the atomic level at ambient pressure and room temperature [1-3]. Compared to other surface analytical techniques, there are several reasons for the diversity of STM and STM-based technological applications: STM can achieve lateral and vertical resolutions of 0.1nm and 0.01nm, respectively, i.e., individual atoms and molecules can be resolved. The higher vertical resolution of STM relative to other microscopes also offers advantages with regard to qualitative analysis of surface roughness on a nanometer scale. STM can be performed in different environments, such as vacuum, air, low or high temperature, etc. Samples can even be immersed in water or other solutions under potential control. In most cases, special techniques for sample preparation are not required, and samples remain mostly free of damage. With these advantages, STM is especially suitable for in-situ electrochemical studies, biological studies, and the evaluation of sample surface under various experimental conditions. The other unique feature of the STM is its truly local interaction with the surface under study at the atomic scale rather than the averaged properties of the bulk phase or of large surface area. This allows the study of individual surface adsorbates, surface 16 EXPERIMENTAL defects, surface reconstructions, and adsorption-induced surface reconstructions at unprecedented resolution. Three-dimensional images of the surface and the solid-fluid interface in real space can be obtained in real time, independent of the sample’s periodicity. This capability allows in-situ imaging of some dynamical processes taking place on surfaces and at the solid-fluid interface. Local surface electronic properties such as charge-density waves, the changes of surface barrier and energy gap, as well as spectroscopic images, can be provided by STM. STM can be employed for the modification of a surface and for the manipulation of atoms and molecules through tip sample interactions, opening up the prospects of constructing atomic or molecular scale devices. 2.1.1 Principle of the Scanning Tunneling Microscopy (STM) Scanning tunneling microscope (STM) is a powerful technique for viewing surfaces at the atomic level. It probes the density of states of a material using tunneling current. The basic design of the STM is shown in Fig 2.1. Based on the concept of quantum tunneling, when a sharp conducting tip is brought very close to the metallic or semiconducting surface ( 10Å), the wave functions of the sample will overlap with the wave function of the tip. If a bias voltage V between the tip and surface is applied, the electrons can travel through the energy barrier via a quantum-mechanical mechanism called tunneling to give rise to a current. 17 EXPERIMENTAL Fig 2.1 Schematic view of scanning tunneling microscope (Copyright: Michael Schmid, TU Wien). It consists of scanning tip, piezoelectric controlled scanner, sample-to-tip control, and data processing component. The direction of the electron flow depends on the sign of the applied bias on the sample. For a positive bias, electrons flow from occupied states of the tip to unoccupied states of the sample and the direction is reversed for the opposite polarity. By keeping the tunneling current constant with an electronic feedback circuit, a fixed tip-sample gap distance can be maintained as the tip is scanned laterally across the sample. Plots of the tip height z versus lateral positions x and y can then be generated and such images yield information about the electronic structure and topography of the surface being analyzed. The tunneling current is proportional to the local density of states (LDOS) near the Fermi level. The current is also proportional to the exponential of the separation d between the tip and the sample: I  e 2d . Such relationship gives STM the ability to 18 EXPERIMENTAL study surfaces with high sensitivity to height changes due to individual atoms. Even though the principle of a scanning tunneling microscope is not very complicated, many factors must be taken into consideration in the design to ensure a stable and reliable performance. Optimum functioning of an STM device requires tip-to-sample position control with picometer precision, a fine positioning capability in three dimensions, high scanning speed, and simplicity of operation. These requirements have to be satisfied in the presence of building vibrations with up to micrometer-size amplitudes, electric noise, thermal drift, creep and hysteresis of the piezoelectric translation elements and other perturbations [1, 2]. In our experiment we use the commercial available product Nanoscope IIIa provided by Veeco Asia. This microscope system has both AFM and STM functions which can be operated in air or organic solutions [4]. 2.1.2 Vibration Isolation of the STM The frame of the instrument is always subjected to vibrations transmitted from the ground or the air. Since the tunneling current depends exponentially on the gap between tip and sample, even the smallest vibrations can affect the stability of the instrument. For many materials, especially metals, the atomic corrugations observed in the constant current STM mode will typically be 0.01 nm. Therefore, a good vibration-isolation system is very important for a well-functioning STM. In the process of minimization of the sensitivity of an STM to vibrations from the building, ventilation ducts and people’s motion, primary attention has to be given to the 19 EXPERIMENTAL frequency range between and 100 Hz. Increasing the inherent resonance frequency of the STM body and employing a vibration damping system are two major ways to isolate vibrations. In our STM system, the vibration isolation is achieved by a massive platform rest on inner tubes which are supported by the compressed air as shown in Fig 2.2. Fig 2.2 Picture of a typical working platform of STM in our lab: the scanner is placed on a vibration-isolated platform. The metal cover was used to block the electromagnetic wave. The floating platform was supported by high pressure air gas. On the platform there is a small plate which is supported by the spring, where the scanning tunneling microscope base is located. The base and the STM head are covered with a metal bell to isolate electromagnetic field from the environment. 20 EXPERIMENTAL Furthermore the whole working platform is covered by a huge and heavy acoustic hood. By all these means the vibrations from building and air can be minimized as much as possible [1, 2, 4]. 2.1.3 Preparation of the STM Tips The reliable fabrication of tunneling probe tips is critical for the proper operation of STM. The size, shape and chemical identity of the tip influence not only the resolution and shape of an STM scan, but also the measured electronic structure. The microstructure of the tip is the key to atomic resolution because the tunneling current depends exponentially on the gap distance. It is necessary to have a single site of closest atomic approach for stable operation, as anomalous imaging artifacts will appear when simultaneous tunneling occurs through multiple atoms on the tip. This is commonly referred to as double-tip imaging. STM tips are typically fabricated from metal wires of tungsten (W), platinum-iridium (Pt-Ir), or gold (Au) and sharpened by mechanical grinding, cutting with a wire cutter or razor blade, “controlled” crashing, field emission/evaporation, ion million, fracture, or electrochemical etching [1, 2]. Preparation of Tungsten Tips The preferred method for preparing tungsten STM tips is the electrochemical etching method. There are two ways by which this can be done: Alternating-Current (AC) or Direct-Current (DC) etching according to the applied potential. Each 21 EXPERIMENTAL procedure gives a different tip shape; the AC etched tips have a conical shape and much larger cone angles than the DC etched tips. The hyperboloid-shaped DC etched tips are much sharper than AC etched tips and are preferable for high-resolution STM imaging. Fig 2.3 Schematic view of the etching machine: it consists of ring-shape Pt electrode, power supply, and wire holder where the sample being etched is fastened. The etchant surface just touchs the Pt-ring. Fig 2.3 illustrates the details of the electrochemical cell used in DC etching, which contains 100 mL of 2M NaOH or KOH. The W wire to be etched is placed in the center of the cell and serves as the anode. It is mounted on a micrometer so that its position relative to the surface of the electrolyte can be precisely adjusted. The counter electrode (or cathode) consists of a Pt ring which surrounds the anode. When a DC voltage of 13V is applied to the anode, bubbles can be observed emerging at the cathode/solution interface. The overall electrochemical reaction is 22 EXPERIMENTAL Cathode: 6H2O + 6e-  3H2 (g) + 6OHAnode: W(s) + 8OH-  WO42- + 4H2O + 6eW(s) + 2OH- + 2H2O  WO42- + 3H2(g) SRP = -2.45 V SRP = +1.05 V E0 = -1.43 V where SRP stands for standard reduction potential. The above reaction involves the oxidative dissolution of W to soluble tungstate (WO42-) anions at the anode, and the reduction of water to form bubbles of hydrogen gas and OH- ions at the cathode. Actually, the reaction mechanism is much more complex than indicated by the above equations and the potential required to drive an electrochemical reaction is usually higher than that calculated from standard electrode potentials [1, 5, 6, 7]. Several factors affect the etching process. Due to the surface tension of the aqueous solution, a meniscus is formed around the wire once it is placed into the electrolyte. It is primarily the shape of the meniscus which determines the aspect ratio and overall shape of the tip. The shorter the meniscus is, the smaller the aspect ratio becomes. A low aspect ratio is important in reducing vibration in the tip during scanning. As the reaction proceeds, the change in the surface area of the wire and in the fluid disturbances may result in the variation of the meniscus height. To avoid oddly shaped tips, the meniscus height should be kept at the same position by adjusting the micrometer during etching. Usually, a quick automatic cutoff circuit is used to cut off the potential to avoid over-etching. The cutoff time of the etching has a significant effect on the radius of curvature of the tip: the faster the cutoff time, the sharper the tip. Because OH- is consumed in the reaction, it is necessary to replace NaOH solution periodically. The 23 EXPERIMENTAL tip-drop-off time increases with the decrease of the OH- concentration. The chemically etched tungsten tips are very sharp but can be oxidized easily in air to form tungsten-oxide layer that has much higher resistance than the desired tunneling gap resistance. It leads to tip crashing sometimes. Thus the tungsten tips are not the most suitable tips for our experiment conducted in air, although they work perfectly in our UHV STM system. The etching system we used here is W-TEK purchased from Omicron Technology. Preparation of Pt-Ir Tips Platinum, although a soft metal, is a material preferred over tungsten because it is inert to oxidation. The addition of Ir to form a Pt/Ir alloy adds stiffness while maintaining a chemically inert material. Pt-Ir tips are widely employed, too, particularly in atmospheric and electrochemical environments. Mechanical shearing is the most common approach for fabricating Pt-Ir tips. In spite of the variation in shape, many experiments had proven that atomic resolution can be achieved using the mechanically fabricated Pt-Ir tips. Although resolution requirements are usually not as stringent for highly topographic samples, wide-area scans place unique restrictions on the tip morphology. For such samples, symmetric, controlled-geometry tips with small radii of curvature and high aspect ratios are necessary to minimize the convolution of the tip shape into the acquired image. However, asymmetric or double tips are often formed during mechanical shearing of the Pt-Ir wire, resulting in misleading sample topography. To make the tip as sharper 24 EXPERIMENTAL as possible, we usually cut the Pt-Ir wire with very large seizers, with the angle  equals to 45°. At the end of cutting, we pull the seizers away from the Pt-Ir tip along the wire. This step helps to form a sharper tip at the moment when the tip is broken into two pieces. Usually the tips cut by this method are always quite sharp, and can be readily used in STM experiments. Fig 2.4 Illustration of the tip cutting procedure: the vertical line represents Pt-Ir wire; the crossing line represents the seizers. Angle  equals to 45. When the tip is nearly dropping off from the wire, the seizers should move along the arrow so that a sharp tip could be produced. Meanwhile several in-situ tip treatments were already used at the birth of STM by the inventors: by gently touching the tip with the sample surface, the resolution can be improved; or by exposing the tip to high electric fields, of the order of 109V/cm, the tips become sharpened. There are additional methods to improve the tip sharpness when the experiment is carried out in air. The freshly prepared STM tips are always washed with absolute ethanol to remove the possible organic contents either from the seizers or environment. The tip is engaged onto surface and then withdrawn from surface for several times. Such movements help to remove the possible contaminants 25 EXPERIMENTAL on the tip. Applying a high voltage (~5V) pulse in milliseconds to the tip can also sharpen the tip. However, the control of the voltage is critical and the high electric field at the tip also leads to a high temperature at which the tip may melt and becomes blunter. 2.1.4 Preparation of the STM Samples A small piece of highly oriented pyrolytic graphite (HOPG) crystal was cut and mounted onto a metallic plate with conducting silver paste. The conductivity between the top of the graphite and the bottom metal plate was tested to ensure that the whole piece of sample is conductive. The metal plate must be magnetic so that it can be secured onto the scanner. The top several layers of graphite were removed by adhesive tapes. The solution containing the molecules to be studied was heated up to around 60ºC. Then one drop of the warm solution was added onto the freshly cleaved graphite surface. The crystallization of molecules was allowed to take place at room temperature before the sample was studied using STM. Fig 2.5 Diagram of a prepared sample: black part is the HOPG crystal adhered to metal plate using conductive silver paste. On top of the graphite there is a drop of depositing sample solution. 26 EXPERIMENTAL 2.1.5 Chemicals The chemicals used consist of 29H,31H-phthalocyanine (Aldrich), copper phthalocyanine (Aldrich), cobalt phthalocyanine (Aldrich), zinc phthalocyanine (Aldrich), heneicosanoic acid (TCI), lignoceric acid (TCI), stearic acid (TCI), and oleic acid (Aldrich), which are all research grade chemicals (98-99% purity) and were used without further purification. 1-phenyloctane (Merck, >98%), 1-heptanol (Aldrich, 98%), 1,2,4 trichlorobenzene (HPLC grade, Aldrich) and toluene (HPLC grade, Aldrich) were used as solvent without further distillation. Samples of 3-(decyloxy)-benzenamine, 3-(tetradecyloxy)-benzenamine, 3(dodecyloxy)-benzenamine, 1-(hexadecyloxy)-2-nitrobenzene were synthesized by the research group under Prof Hardy Chan Sze On of Department of Chemistry, National University of Singapore (NUS). The samples were purified by column chromatography. SU-CY4F, SUCYBB, SU1876 and SU-DT were synthesized and purified by Dr Shu Lijin. Their structures are attached in Scheme 2.1 at the end of this chapter. The osmium organometallic compound Os3H(CO)10S(CH2)10CO2H was provided by Prof Leong Weng Kee of Department of Chemistry, NUS. Polymer samples and porphyrin derivatives were provided by Prof Lai Yee Hing’s group. The structures of the polymers are listed in Scheme 2.2. Samples provided by Prof Valiyaveettil Suresh’s group are listed in the Scheme 2.3. The standard HOPG crystals are of the ZYB grade and were provided by the 27 EXPERIMENTAL Veeco Asia Pte Ltd, Singapore. The silver conductive paint was purchased from R.S Components, Northants, UK. 2.2 Computational Studies - Forcite The Forcite program packages in Materials Studio (version 4.0) of Accelrys Inc were employed for the computational works in the current project. This is a molecular mechanics module for potential energy and geometry optimization calculation of arbitrary molecular and periodic systems using classical mechanics. It offers support for the COMPASS, UFF, and Dreiding forcefields. With this wide range of forcefields, Forcite can handle essentially any material. The geometry optimization algorithm offers steepest descent, conjugate gradient, quasi-Newton, and full Newton-Raphson Methods, in addition to the Smart algorithm, which uses these methods sequentially. This allows very accurate energy minimization to be performed [8]. As the most molecular clusters in our project contain several layers of graphite, the number of atoms in each module is usually more than 300. The construction and calculation of the adsorption energies of the molecules on the graphite (0001) surface are based on the clusters having a planar molecule positioned above the surface with a distance larger than a C-C single bond (>1.54 Å). In most cases the distance was set at 2.0 Å. The way of positioning molecules on the graphite is based on the STM results. All carbon atoms of the graphite were constrained to represent the bulk-like environment. The adsorption energies, Ead, for different configurations were calculated by 28 EXPERIMENTAL subtracting the energies of the cluster comprising the adsorbate molecules and the substrates from the total energies of the free substrate cluster and the gas-phase adsorbates as shown in Equation 2.1: Ead = E(surface) + E(adsorbate) - E(adsorbate/surface) (2.1) With this definition, a positive Ead corresponds to stable adsorption on the surface. For the calculations, the Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) forcefield was employed for both geometry optimization and dynamics. For the convergence tolerance, the Medium quality was chosen with energy at 0.001 kcal/mol and force of 0.5 kcal/mol/Å. In the dynamics calculation the temperature was set at room temperature 298K. 29 EXPERIMENTAL References [1] Wiesendanger, R. Scanning Probe Microscopy and Spectroscopy Methods and applications, Cambridge University Press, 1994. [2] Bai, C.L., Scanning tunneling microscopy and its application, Scientific Technical Publishers, 1995. [3] Drake, B.; Sonnenfeld, R.; Schneir, J.; Hansma, P.K.; Slough, G.; Coleman, R.V. Rev. Sci. Instrum. 1986, 57, 441. [4] Digital Instrument Veeco Metrology Group, MultiModeTM SPM Instruction Manual, Version 4.31ce, 1996. [5] Zhang, B.L.; Wang, E.K. Electrochimica Acta, 1994, 39, 103. [6] Nagahara, L.A.; Thundat, T.; Lindsay, S.M. Rev. Sci. Instrum. 1989, 60, 3128. [7] Kazinczi, R.; Szöcs, E.; Kálmán, E.; Nagy, P. Appl. Phys. A 1998, 66, S535. [8] Forcite - Materials Studio Online Help Manual 30 EXPERIMENTAL Scheme 2.1 Chemical structures of SUCYBB, SU-CY4F, SU1876 and SU-DT (by Dr Shu Lijin): SU-CYBB F F F F F F F F SU-CY4F 31 EXPERIMENTAL C12H25 Si SU-1876 triisopropyl(2-(4-(4-(4-(tetradec-1-ynyl)phenyl)buta-1,3-diynyl)phenyl)ethynyl)silane Si Si SU-DT 1,4-bis(4-(2-(triisopropylsilyl)ethynyl)phenyl)buta-1,3-diyne 32 EXPERIMENTAL Scheme 2.2 Molecules provided by Prof Lai Yee Hing’s group: OC6H13 O O n y x C6H13O x y Sample Sample Sample Br Br NH N N HN Br Br NH N N HN 33 EXPERIMENTAL Scheme 2.3 Derivatives of hexabenzocoronene by Prof Valiyaveettil Suresh’s group C12H25 C12H25 C12H25 N C12H25 C12H25 C12H25 N C12H25 C12H25 C12H25 C12H25 CPAH CHBC C12H25 Br N Br N C12H25 C12H25 C12H25 CHBC2 N C12H25 N C12H25 TCC 34 EXPERIMENTAL R N R N H3CS SCH3 N R R SCH3 N N N N N R N R R=CH2C(CH3)C4H9 R=CH2CH(C2H5)C4H9 TCC2 TCC1 TpA R N R N N N R N R S N N N R N R S S R=CH2CH(C2H5)C4H9 TCC5 R=CH2CH(C2H5)C4H9 TCC4 TpA N C H 12 25 N C12H25 N C12H25 SivaCyc2 35 EXPERIMENTAL C12H25 C12H25 N C12H25 C12H25 N C12H25 C12H25 CHBC3 Derivatives of perylene by Prof Valiyaveettil Suresh’s group C12H25 N O O O C12H25 N O O C12H25 N O S Br Br S O N O C12H25 O O DDPER N O C12H25 S170 S171 O N O C12H25 C12H25 N O O C12H25 N O OMe OMe MeO OMe MeO MeO OMe OMe O N O C12H25 S169 O N O C12H25 S172 36 EXPERIMENTAL O C12H25 N O O C12H25 N O OCH3 OCH3 H3CO S S H3CO OCH3 OCH3 O N O C12H25 SIV233 O N O C12H25 SIV236 37 [...]... hexabenzocoronene by Prof Valiyaveettil Suresh’s group C12H25 C12H25 C12H25 N C12H25 C12H25 C12H25 N C12H25 C12H25 C12H25 C12H25 CPAH CHBC C12H25 Br N Br N C12H25 C12H25 C12H25 CHBC2 N C12H25 N C12H25 TCC 34 EXPERIMENTAL R N R N H3CS SCH3 R N N R N SCH3 N N N R N R R=CH2C(CH3)C4H9 R=CH2CH(C2H5)C4H9 TCC2 TCC1 TpA R N R N N N R N R S N N N R N R S S R=CH2CH(C2H5)C4H9 TCC5 R=CH2CH(C2H5)C4H9 TCC4 TpA N C H 12 25... 12 25 N C12H25 N C12H25 SivaCyc2 35 EXPERIMENTAL C12H25 C12H25 N C12H25 C12H25 N C12H25 C12H25 CHBC3 Derivatives of perylene by Prof Valiyaveettil Suresh’s group C12H25 N O O O C12H25 N O O C12H25 N O S Br Br S O N O C12H25 O O DDPER N O C12H25 S170 S171 O N O C12H25 C12H25 N O O C12H25 N O OMe OMe MeO OMe MeO MeO OMe OMe O N O C12H25 S169 O N O C12H25 S1 72 36 EXPERIMENTAL O C12H25 N O O C12H25 N O OCH3... melt and becomes blunter 2. 1.4 Preparation of the STM Samples A small piece of highly oriented pyrolytic graphite (HOPG) crystal was cut and mounted onto a metallic plate with conducting silver paste The conductivity between the top of the graphite and the bottom metal plate was tested to ensure that the whole piece of sample is conductive The metal plate must be magnetic so that it can be secured onto... number of atoms in each module is usually more than 300 The construction and calculation of the adsorption energies of the molecules on the graphite (0001) surface are based on the clusters having a planar molecule positioned above the surface with a distance larger than a C-C single bond (>1.54 Å) In most cases the distance was set at 2. 0 Å The way of positioning molecules on the graphite is based on. .. Northants, UK 2. 2 Computational Studies - Forcite The Forcite program packages in Materials Studio (version 4.0) of Accelrys Inc were employed for the computational works in the current project This is a molecular mechanics module for potential energy and geometry optimization calculation of arbitrary molecular and periodic systems using classical mechanics It offers support for the COMPASS, UFF, and Dreiding... Prof Hardy Chan Sze On of Department of Chemistry, National University of Singapore (NUS) The samples were purified by column chromatography SU-CY4F, SUCYBB, SU1876 and SU-DT were synthesized and purified by Dr Shu Lijin Their structures are attached in Scheme 2. 1 at the end of this chapter The osmium organometallic compound Os3H(CO)10S(CH2)10CO2H was provided by Prof Leong Weng Kee of Department of. .. 0.5 kcal/mol/Å In the dynamics calculation the temperature was set at room temperature 29 8K 29 EXPERIMENTAL References [1] Wiesendanger, R Scanning Probe Microscopy and Spectroscopy Methods and applications, Cambridge University Press, 1994 [2] Bai, C.L., Scanning tunneling microscopy and its application, Scientific Technical Publishers, 1995 [3] Drake, B.; Sonnenfeld, R.; Schneir, J.; Hansma, P.K.;... results All carbon atoms of the graphite were constrained to represent the bulk-like environment The adsorption energies, Ead, for different configurations were calculated by 28 EXPERIMENTAL subtracting the energies of the cluster comprising the adsorbate molecules and the substrates from the total energies of the free substrate cluster and the gas-phase adsorbates as shown in Equation 2. 1: Ead = E(surface)... E(adsorbate/surface) (2. 1) With this definition, a positive Ead corresponds to stable adsorption on the surface For the calculations, the Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) forcefield was employed for both geometry optimization and dynamics For the convergence tolerance, the Medium quality was chosen with energy at 0.001 kcal/mol and force of 0.5 kcal/mol/Å... scanner The top several layers of graphite were removed by adhesive tapes The solution containing the molecules to be studied was heated up to around 60ºC Then one drop of the warm solution was added onto the freshly cleaved graphite surface The crystallization of molecules was allowed to take place at room temperature before the sample was studied using STM Fig 2. 5 Diagram of a prepared sample: black . Scheme 2. 3 Derivatives of hexabenzocoronene by Prof Valiyaveettil Suresh’s group C 12 H 25 C 12 H 25 C 12 H 25 C 12 H 25 N C 12 H 25 CPAH C 12 H 25 C 12 H 25 C 12 H 25 C 12 H 25 N C 12 H 25 CHBC . project contain several layers of graphite, the number of atoms in each module is usually more than 300. The construction and calculation of the adsorption energies of the molecules on the graphite. C 12 H 25 C 12 H 25 C 12 H 25 C 12 H 25 N C 12 H 25 CHBC Br Br C 12 H 25 C 12 H 25 N C 12 H 25 CHBC2 N N N C 12 H 25 C 12 H 25 C 12 H 25 TCC 34