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Conducting Probe Atomic Force Microscopy Investigation of Anisotropic Charge Transport in Solution Cast PBD Single Crystals Induced by an External Field Minlu Zhang, Zhijun Hu, and Tianbai He* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ReceiVed: April 24, 2004; In Final Form: September 20, 2004
2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxdiazole (PBD) is a good electron-transporting material and can form single crystals from solution. In this work, solution cast PBD single crystals with different crystallographic axes (b, c) perpendicular to the Au/S substrates in large area are achieved by controlling the rate of solvent evaporation in the presence and absence of external electrostatic field, respectively. The orientation of these single crystals on Au/S substrate was characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Conducting probe atomic force microscopy (CP-AFM) was used to measure the charge transport characteristics of PBD single crystals grown on Au/S substrates. Transport was measured perpendicular to the substrate between the CP-AFM tip and the Au/S substrate. The electron mobility of 3 × 10-3 cm2/(V s) for PBD single crystal along crystallographic b-axis is determined. And the electron mobility of PBD single crystal along the c-axis is about 2 orders of magnitude higher than that along the b-axis due to the anisotropic charge transport at the low voltage region. We demonstrate that CP-AFM may be applied successfully to measuring the anisotropic charge transport of single crystals over nanoscopic length scales.
Introduction Organic semiconducting materials have been widely studied due to their various merits for easy processing (e.g., printing, solution casting), good compatibility with many substrates including flexible plastics, and great opportunities in structural modifications.1 Most studies have been made in the field of highly ordered organic thin films that have been shown to exhibit many promising properties due to their wide applications to electrical and optical devices.2 The performance of such devices is strongly dependent on the degree of ordering of the molecules, grain boundaries, and interface effects in polycrystalline thin film. Recently, single-crystal based organic thin-film transistors have been extensively studied,3 because intrinsic electrical properties can be more directly studied in bulk single crystalline materials where the influence of grain boundaries, residual disorder, and interface effects is minimized. It has been reported that the pentacene device demonstrated a performance similar to that of amorphous Si (R-Si) with hole mobilities exceeding 1.5 cm2/(V s).4 Its high performance has been partly due to the ability of forming single-crystal-like films. Conducting probe atomic force microscopy (CP-AFM) has been proved to be an effective means for investigating the electrical properties of organic crystal thin film, especially single crystal organic materials within the nanoscale. Both nanoscale electrical characterization and topographic imaging can be studied particularly well by CP-AFM because CP-AFM employs a metal-coated cantilever-tip assembly as both a scanning electrical contact and force sensor and can directly measure the I-V of the tip position relative to the samples that are highly resistive or surrounded by insulating regions.5a,b Previous studies revealed that the electrical measurements of various materials * To whom all correspondence should be addressed. E-mail: tbhe@ ciac.jl.cn. Fax: +86-431-5262126. Tel: +86-431-5262123.
such as carbon nanotubes (CNTs),5c,d organic crystal/molecule,5a,b,e-i polymer blends,5j,k and gold nanowire5l could be performed by CP-AFM. Frisbie et al.5a had focused on the investigation of sexithiophene (6T) crystals from the vertical and horizontal direction by CP-AFM over nanometer length scale. It has been shown that 6T crystals exhibit electrical anisotropy: linear in the (50 mV regime in perpendicular studies and nonlinear but rectifying in a larger voltage range in parallel studies. Due to the anisotropic molecules the electrical properties of single crystals are also anisotropic. Thus, the electrical anisotropy as a fundamental property of these molecular crystals can be studied particularly well in single crystals. Although organic thin films have been grown previously by using conventional physical vapor deposition, the advantages (i.e., processability and low-cost) of organic materials in device applications can be truly realized through liquid-phase processing techniques such as solution casting, or printing.1 The oligomer-based compounds that are liquid phase processible can form a well ordered film from solution with relatively high performance.1 The best mobility ca. 0.03 cm2/(V s) has been achieved with DH-R-6T from chlorobenzene solution.6 Additionally, solution processible polymeric semiconductors, such as poly(3-hexylthiophene), have hit mobilities of 0.1 cm2/(V s).7 It is relatively difficult to obtain a single crystal by physical vapor deposition, not to mention to manipulate the orientation of the single crystal during the deposition. However, organic compounds can easily form single crystals from solution, which is a relatively more convenient way to obtain single crystalline materials and can be manipulated by an external field such as an electrical field8a or magnetic field,9 etc. In this paper, we measure the electrical anisotropy of PBD (Chart 1A) single crystals, which were obtained in large area from solution cast and induced by two external fields, by CP-AFM. PBD has been studied extensively,8b,10 because it is
10.1021/jp0482058 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/17/2004
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CHART 1: (A) Chemical Structure of PBD; (B) Configuration for the Vertical I-V Measurements
known be a good scintillator dye and has good electron transport properties. It has been reported that the solvent field is a strong, high directional field.11 In addition, the electric field is another external field that can manipulate the orientation of small molecules,8 etc. We first obtained PBD single crystals with the crystallographic c-axis perpendicular to the substrates by controlling the rate of solvent evaporation without electrostatic field, and PBD single crystals with the b-axis perpendicular to the substrates were achieved by controlling the rate of solvent evaporation with electrostatic field. CP-AFM was used to measure the anisotropic charge transport through the thickness of PBD single crystals, Chart 1B. To make reproducible electrical measurements, we first use a point-contact mode (for measuring I-V characteristics) to measure the I-V curves before imaging, and then a contact mode (for topographic images) is used to image the topography without moving the coordinates. Experimental Section
Figure 1. (A) and (B) SEM images of an Au-coated CP-AFM tip.
Materials. PBD (C24H22N2O) and NaSH were purchased from Aldrich and used as received. Preparation of Conducting Substrates. The silicon wafers were cut into 1.4 cm × 1.4 cm squares and then were cleaned by heating at 90 °C in a mixture of 30% H2O2 and concentrated H2SO4 (30:70 v/v) for 30 min rinsed with plenty of clean water and dried with a stream of nitrogen prior to Au deposition. Gold films were deposited onto Si substrates using thermal evaporation at a background pressure of 3 × 10-6 Torr. Typically, 700 Å of gold was deposited on a 30 Å Cr adhesion layer at 1 Å/s. The Au substrates were cleaned by immersing briefly in hot (>100 °C) concentrated sulfuric acid, rinsing thoroughly with enough purified water, and then cleaned by sonication in ethanol for 20 min, respectively. These Au substrates were immersed for 30 s in a 1 mM aqueous solution of NaSH (filtered). Before and after this process, Au substrates were gently blown with dry nitrogen. We use this pretreatment of Au substrate with S (referred to as Au/S) reported by Frisbie5a to make S-terminated surface chemically similar to SiO2 to promote crystallite formation and this treatment will not appreciably influence the conductivity of the Au substrates. Preparation of Single Crystals. The PBD powder sample was dissolved in chloroform in a concentration of 0.001% w/v. One droplet was deposited onto Au/S substrates at room temperature. A slow solvent evaporation rate was achieved by placing the solution-cast films inside a cylinder container of radius and height 1 and 2.5 cm, respectively, covered with a lid. Under these conditions, solvent can only escape through the small gap between the container and its lid. The evaporation of solvent in the solution was complete after approximately 12
h for the container maintained at room temperature, then the sample was taken out for observation. When applying the electrical field, we placed the whole container with a sample under an electrostatic field with an intensity of 5 kV/cm at room temperature under ambient conditions for about 12 h. Instruments. Conducting Probe Atomic Force Microscopy. CP-AFM was performed using an SPA-300HV atomic force microscope (AFM) with a SPI 3800N controller (Seiko Instruments Industry Co., Ltd.). A metal spring clip that holds the cantilever substrate in place was used to make electrical contact to the Au probe. A second contact was made to the sample substrate connected with a plot of Au substrate by silver paint. The silver paint was purchased from SPI. To make an electrical measurement, we first used a point-contact mode for measuring I-V characteristics and then imaged the sample with contact mode without moving the sample after I-V characterization. The Au-coated AFM probe purchased from Seiko Instruments Industry Co., Ltd. was used for current detection, and its force constant and resonant frequency were 0.11-0.12 N/m and 13 kHz, respectively. The applied load was 30 nN to observe the I-V characteristics of PBD single crystals. The tip radius of the cantilever was about 100 nm.12 Figure 1A,B shows scanning electron microscopy (SEM) images of Au-coated probes at two different magnifications. Electrical Field. The instrument of the electrostatic field used was designed in our laboratory. Adjustable distances and voltages between the top and bottom electrode could be controlled for changing the electrostatic field intensity. Transmission Electron Microscopy. Transmission electron microscopy (TEM) experiments were performed using a JEOL
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2010 TEM with an accelerating voltage of 200 kV for bright field and electron diffraction (ED) modes. Before observation, the single crystals deposited on Au/S substrates were transferred from the substrate in water and collected on a carbon-coated copper TEM grid. Calibration of the electron diffraction spacing was carried out using Au. Results and Discussion Orientation of PBD Single Crystals. In the previous study,13 Qin has shown that PBD formed a polycrystalline thin film and few single crystals from solution cast without controlling the rate of solvent evaporation. The crystallographic c-axis of these polycrystalline thin films and single crystals was perpendicular to the substrate, preferably. The PBD crystals grow in an orthorhombic lattice structure in the P212121 space group, and the unit cell lattice constants are a ) 6.439 Å, b ) 7.975 Å, and c ) 37.415 Å.13 Figure 2 shows the stereographic view of the PBD unit cell from a and b axes, respectively. It is unlike 6T whose molecules pack in the herringbone fashion, because the PBD molecule is not planar due to the three dihedral angles of 26.57°, 11.15°, and 5.678° that exist between the two adjacent benzene rings, between 1,3,4-oxadiazole and benzene, and between 1,3,4-oxadiazole and benzene, from left to right, respectively, and because the tert-butyl increases the steric hindrance effect.13 Assuming the axis of 2-(4-biphenylyl) as the molecular long axis, the molecular long axis is at an angle of 103.6° with a-axis, 28.3° with b-axis and 65.6° with c-axis, calculated by SHELXTL software. It is expected that the molecules are nearly lying in the ab plane with an angle of 24.4° and almost standing in the ac plane with an angle of 61.7°. Due to the kinetically rapid process of crystallization, few single crystals are formed. However, when the speed of solvent evaporation was controlled at a slow rate, PBD is allowed to slowly crystallize during the solvent evaporation, and rodlike single crystals formed on Au/S substrates were observed by atomic force microscopy in Figure 3. When the rate of solvent evaporation was decreased, PBD formed separated single crystals along certain direction and this growth proceeds close to kinetic equilibrium. Consequently, the organic molecules can find energetically the most suitable arrangement before being incorporated into the crystal lattice and preferential orientation of the molecules is expected. The crystals on Au/S substrate typically range from 10-70 nm in thickness. We found that passivation of the flat Au substrates with S atoms (by exposure to aqueous NaSH) was helpful in growing thin PBD single crystals and will not influence the structure of PBD single crystals, which was verified by Frisbie.5a Figure 4 shows AFM height image and corresponding ED pattern of the rodlike PBD crystals with an area of 30 × 30 µm2 (Figure 4A inset) obtained by controlling the rate of solvent evaporation on Au/S substrate. TEM indicated that both kinds of PBD crystals were single crystals. The height image in Figure 4A shows an individual PBD single crystal with multiple terraces on an Au/S substrate. The terrace steps are approximately 3.77 nm tall, as shown in the line scan in Figure 4B, corresponding to approximately the value of the c parameter of the unit cell. According to Qin,13 the PBD single crystal shows the hk0 reflection series in the ED pattern, which indicates that the PBD single-crystal grows with the crystallographic c-axis perpendicular to Au/S substrate. The terrace steps in Figure 4A help to indicate that the crystallographic c-axis is perpendicular to the Au/S substrate, indicating that the PBD molecules are nearly lying with an angle of 24.4° on the Au/S substrate (referred to as l-PBD).
Figure 2. Stereographic view of the PBD unit cell:13 (A) view from a-axis; (B) view from b-axis.
Figure 5 shows the AFM image and ED pattern of the rodlike PBD single crystal with an area of 30 × 30 µm2 (Figure 5A inset) induced by an electrostatic field. We find that the morphology is the same as those obtained without applying an electrostatic field, but the ED pattern is different. The height image in Figure 5A shows an individual PBD single crystal with multiple terraces on an Au/S substrate. The terrace steps are approximately 0.72 nm tall, as shown in the line scan in Figure 5B, corresponding to approximately the value of the b parameter of the unit cell. According to Qin,13 the PBD singlecrystal achieved by electrostatic field shows the h0l reflection series in the ED pattern in Figure 5C, which indicates that the PBD single crystals grow with the crystallographic b-axis perpendicular to the Au/S substrate. All these results indicate that PBD molecules are standing on the Au/S substrate with an angle of 61.7° (referred to as s-PBD). The dipole moment of
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Figure 3. AFM height images of PBD single crystals grown by solution cast with slow rate of solvent evaporation on Au/S substrates.
the PBD molecule is definitely distinct from zero because the dipole moment of the analogous 2-(4-biphenylyl)-5-(phenyl)1,3,4-oxdiazole molecule is 3.66 D.8b,14 We find that the dipole moment of the 1,3,4-oxdiazole molecule is about 2.75-3.15 D with the direction of the dipole moment parallel to the N(3)-N(4) band;15a,b the dipole moment of tert-butylbenzene is only about 0.7 D15c and biphenyl is a nonpolar molecule. Because the permanent dipole moment is determined by the detailed chemical structure of molecules in which each polar chemical bond possesses a dipole,16 we can conclude that the permanent dipole moment of PBD is more or less parallel to the molecular long axis, at least not perpendicular to the molecular long axis. When induced by an electrostatic field, the PBD single crystals grown with molecules almost standing on the substrate were obtained, and we control the rate of solvent evaporation to manipulate the molecules with enough time by the electrostatic field. CP-AFM Characterization of PBD Single Crystals. The electrical transport in PBD single crystals was studied by the measurement of current-voltage (I-V) characteristics in Au/PBD/Au devices. Figures 6 and 7 show the I-V characteristics of devices based on s-PBD and l-PBD single crystals, respectively. We observed nonlinear I-V traces for s-PBD single crystals in the (4 V regime, but linear traces for l-PBD single crystals in the (20 mV regime because if we pass too much current the tip will blow up. As shown in Figure 6B, where log-log plots of I-V characteristics of Au/s-PBD/Au (d ) 49 nm) were given, the I-V curve includes three regions, significantly. It was found that the injection current is proportional to the square of the voltage, i.e., j ∝ V2, whereas the thickness dependence of the current is also satisfied with j ∝ d-3, as shown in the inset of Figure 6B at the high voltage region. The observed thickness and voltage dependence demonstrates that the carrier transport in s-PBD single crystals can be described by spacecharge-limited current (SCLC) models with single discrete level traps, and the current density is written17
9 V2 J ) 0θµ 3 8 d
(1)
where and 0 are the dielectric permittivity, and
θ)
n n + nt
or
θ)
p p + pt
(2)
for electrons and holes, respectively (n is the density of free
Figure 4. Morphology and structural characterization of l-PBD single crystals on Au/S substrate. (A) AFM height image of an l-PBD single crystal showing multiple terraces. Inset: AFM height image of an l-PBD single crystal with total scanning area of 30 × 30 µm2. (B) Height profile along solid line in (A) showing a 3.77 nm step. (C) Electron diffraction pattern of the single crystal with a proper crystallographic orientation.
electrons, nt is the density of trapped electrons, p is the density of free holes, pt is the density of trapped holes). In this case, θµ ≡ µeff is the effective charge carrier mobility. According to eq 1, the effective charge carrier mobility can be readily determined by the space-charge-limited current measurements. Because the tip radius is about 100 nm, by assuming ) 3, a value of µeff ) 3 × 10-3 cm2/(V s) was obtained for an s-PBD single crystal. The result being significantly higher than reported PBD [(2-4) × 10-5 cm2/(V s)]10b,c is a consequence of charge transport in a single crystal. Figure 7A shows the I-V curves of Au/PBD/Au devices for l-PBD single crystals with different thicknesses. Figure 7B
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Figure 6. (A) Point contact I-V characteristics of an s-PBD single crystal obtained by CP-AFM with different thickness. (B) log-log plots of I-V characteristics of Au/s-PBD/Au (d ) 49 nm). Region (a) corresponds to ohmic injection (j ∝ V). Region (b) corresponds to the injection of electrons and region (c) to j ∝ V2. Inset: I(d) dependence at V ) 2.73 V.
Figure 5. Morphology and structural characterization of s-PBD single crystals on Au/S substrate. (A) AFM height image of an s-PBD single crystal showing multiple terraces. Inset: AFM height image of an s-PBD with total scanning area of 30 × 30 µm2. (B) Height profile along solid line in (A) showing a 0.72 nm step. (C) Electron diffraction pattern of the single crystal with a proper crystallographic orientation.
shows log-log plots of I-V characteristics of Au/l-PBD/Au (d ) 34 nm). It can be seen that an ohmic behavior was observed, j ∝ V. The approximate characteristics were also observed in the case of an s-PBD single crystal at the low voltage region, as shown in region (a) of Figure 6B. Figure 8 compares the current-voltage curves of l-PBD and s-PBD in the low voltage region; the current through an l-PBD single crystal is about 2 orders of magnitude higher than that through an s-PBD single crystal. The difference is attributed to the anistropic charge transport. And the carrier mobility in an l-PBD single crystal is at least 2 orders of magnitude higher than that in an s-PBD single crystal. PBD is an electron-transporting
material, and it is generally considered that the electron mobility is much larger than the hole mobility in a PBD single crystal. The LUMO and HOMO energy levels of PBD are -2.4 and -5.9 eV, respectively,10c and an Au work function was -5.1 eV. For the case of Au/PBD/Au device, the barrier height to hole and electron injection is 0.8 and 2.7 eV, respectively. Although the holes have a lower barrier height than the electrons, for the electron-type organic molecule PBD we consider that the injection is electrons at high voltage. The increase from low voltage to high voltage is due to the injection of the electrons. Therefore, we conclude that the determined mobility is electron mobility. Due to the anisotropic single-crystal structure, the electrical properties of PBD single crystals are also anisotropic. As shown in Figure 8, the conductivity of l-PBD and s-PBD (obtained from the ohmic region) exhibits a strong anisotropy, resulting in different mobilities. It is generally believed that charge transport in organic materials is realized by π-π interactions between molecules through a hopping mechanism. During the hopping process, charges transport preferentially along the stacking axis of molecules through their overlapping of π orbitals, and it has been shown that structural organization of molecules in solids plays an important role because the overlap of π orbitals of neighboring molecules is dependent on the interstacking distances.2a,b The PBD molecule is not planar because three dihedral angles exist in the molecule chain. However, π-delocalization is across the whole molecule chain.18 When PBD molecules are stacking along the c-axis, the interstacking distance between the neighboring molecules is relatively smaller than when the PBD molecules are stacking along the b-axis, which is approximately the value of the b-axis, as shown in Figure 2A. When carriers are injected into PBD
AFM of Charge Transport in PBD Single Crystals
J. Phys. Chem. B, Vol. 108, No. 50, 2004 19203 or standing on the substrates in a large area in the absence and presence of an external electrostatic field, respectively. The orientation of PBD single crystals was achieved with a slow rate of solvent evaporation. We also presented that CP-AFM may be applied successfully to measuring the anisotropic charge transport of single crystals over nanoscopic length scales. Our experimental results demonstrate that the Au/s-PBD/Au is spacecharge-limited at the high voltage region. The electron mobility of 3 × 10-3 cm2/(V s) for a PBD single crystal along b-axis is directly determined by using space-charge-limited current analytical expressions. The electron mobility of a PBD single crystal along the c-axis is about 2 orders of magnitude higher than that along the b-axis, determined by the anisotropic charge transport in the low voltage region. It is quite possible that the much higher µeff is a consequence of charge transport in a single crystal. The anistropic charge transport is approximately 100, which is the result of the different overlap of π orbitals due to the different stacking of molecules. Acknowledgment. We thank Prof. D. Ma at Changchun Institute of Applied Chemistry for fruitful discussion. This work was supported by the National Science Foundation of China. Supporting Information Available: List of PBD singlecrystal crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 7. (A) Point contact I-V characteristics of an l-PBD single crystal obtained by CP-AFM with different thickness. (B) log-log plots of I-V characteristics of Au/l-PBD/Au (d ) 34 nm). The continuous line is fitted to j ∝ V (ohmic). Inset: I(d) dependence at V ) 0.1 V.
Figure 8. Current vs applied electrical fieldcharacteristics of anisotropic electrical conduction for PBD single crystals in ohmic region.
single crystals along the c-axis, the closest interstacking distance causes a bigger overlap of the π molecular orbital in planes. However, when carriers are injected into the PBD single crystal along the b-axis, a relatively larger interstacking distance causes a smaller overlap of the π molecular orbital in planes. Therefore, the overlap of π orbitals in a PBD single crystal along the c-axis is larger than that along the b-axis, resulting in the higher conductivity of l-PBD single crystals. That the mobility of PBD single crystals with molecules nearly lying on the substrates is better than those with molecules almost standing is consistent with p-sexiphenyl2e,f and PBD doped in PS,8b the large anisotropy is due to the anisotropy of the single crystals structure. Conclusions We demonstrated that by controlling the rate of solvent evaporation and applying an electrostatic field we obtained solution cast PBD single crystals with molecules almost lying
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