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Origin of Photochemical Modification of the Resistivity of Ag(DMe-DCNQI)2 Studied by X-ray Absorption Fine Structure Takeshi Miyamoto,*,†,‡ Yoshinori Kitajima,§ Hideyuki Sugawara,| Toshio Naito,| Tamotsu Inabe,| and Kiyotaka Asakura*,†,‡ Department of Quantum Science and Engineering, Graduate School of Engineering, Hokkaido UniVersity, Kita21 Nishi10, Kita-ku, Sapporo 001-0021, Japan, Catalysis Research Center (CRC), Hokkaido UniVersity, Kita21 Nishi10, Kita-ku, Sapporo 001-0021, Japan, Institute of Materials Structure Science, Photon Factory, High Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba 305-0801, Japan, and DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Kita10 Nishi8, Kita-ku, Sapporo 060-0810, Japan ReceiVed: July 23, 2009; ReVised Manuscript ReceiVed: October 10, 2009
The Ag(DMe-DCNQI)2 (DMe-DCNQI ) 2,5-dimethyl-N,N′-dicyanoquinonediimine, or DM) charge-transfer salt is a promising material for production by photolithography as it displays a unique photoinduced change in conduction. Photoproducts (β1 and γ) of Ag(DM)2 were investigated using the X-ray absorption fine structure (XAFS) technique in order to understand the origin of their conduction properties. In contrast to the metallic conduction exhibited by a pristine sample (R), β1 is a semiconductor, whereas γ is an insulator, even though the original Ag(DM)2 composition is maintained in both the β1 and the γ forms. A redox mechanism has been postulated to explain the photoinduced change in conduction. However, measurement of the Ag L3edge XANES (X-ray absorption near-edge structure) did not provide any evidence of electron transfer from the DM radical anion to the Ag cation in the β1 and γ products. Ag K-edge extended X-ray absorption fine structure (EXAFS) data demonstrated that β1 is a mixture of the original pristine R phase and a newly found R′ phase, which has a shorter bond distance between the Ag cations and the N atoms of DM molecules. The conduction electrons remain in the DM column of R and R′, but the domain boundaries between R and R′ present an activation barrier for the conduction electrons to cross these boundaries, which provides a rationale for the semiconductive behavior of β1. The Ag K-edge EXAFS results showed that the γ photoproduct has a different local structure from R, with a shorter distance and smaller coordination number of the Ag-N bond. The DM radical anions in the γ compound form covalent bonds between themselves, resulting in a loss of the columnar structure. The complex local structure around Ag cations explains the insulating behavior of the γ compound. 1. Introduction Organic electronics have unique functions that are difficult to achieve with silicon-based devices. Lightness, flexibility, low cost, and ease of device fabrication in large area are advantages of organic electronics that have resulted in much attention being given to organic devices.1-3 Organic devices can be produced from several types of organic materials, that is, small molecules, polymers, and dendrimers, by deposition or printing techniques for the fabrication of junctions and circuits.4-7 If the conduction properties of a single organic electronic material can be controlled by artificial means, then various types of organic electronic devices could be fabricated using just one organic material. However, the chemical doping methods used in conventional semiconductor device fabrication technologies, such as ion implantation and plasma processes, are not suitable for control of the conduction properties of organic conductors due to the serious damage they cause. The Ag(DMe-DCNQI)2 (DMe-DCNQI ) 2,5-dimethyl-N,N′-dicyanoquinonediimine, or DM; Figure 1) charge-transfer salt shows an irreversible change * To whom correspondence should be addressed. Tel: +81-11-706-9114 (T.M.), +81-11-706-9113 (K.A.). E-mail:
[email protected] (T.M.),
[email protected] (K.A.). † Graduate School of Engineering, Hokkaido University. ‡ Catalysis Research Center (CRC), Hokkaido University. § High Energy Accelerator Research Organization (KEK). | Graduate School of Science, Hokkaido University.
in its resistivity with UV-vis irradiation under ambient conditions,8,9 which makes it a candidate material for the photolithographic control of conductivity. The resistivity of Ag(DM)2 can be controlled by the intensity and duration of the UV-vis irradiation; a rectifying device has been successfully produced from a single crystal solely by UV-vis irradiation.8,10 Before UV-vis irradiation, Ag(DM)2 in its initial R state is an opaque crystalline solid. The Ag is connected with four DM molecules in a pseudotetrahedral through a coordinate bond. It has one-dimensional (1D) metallic conduction at room temperature (RT).11-17 The 1D metallic conduction of R is mainly due to two factors: (1) DM molecules assemble in a columnar structure along the c axis and interact through π-π overlaps, creating a 1D π band. (2) DM molecules accept unpaired electrons from Ag atoms, which are accommodated in the 1D π orbitals and behave as conduction electrons. Half of the DM molecules become radical anions, and the DM unit has an average formal charge of -0.5 in the R form. Both the DM π band and the delocalized unpaired π electrons are necessary for the compound to exhibit 1D metallic conduction. When the R form is exposed to UV-vis irradiation, the Ag cations should be photoreduced with electrons being back-transferred to the Ag cations, thereby reducing the number of conduction electrons. The intensity and duration of photoirradiation determine the number of remaining conduction electrons, and consequently,
10.1021/jp907024p CCC: $40.75 2009 American Chemical Society Published on Web 11/02/2009
Resistivity of Ag(DMe-DCNQI)2
J. Phys. Chem. C, Vol. 113, No. 47, 2009 20477 ing the electronic states and the local structure around the Ag cations in β1 and γ were obtained from the XAFS results. These results were different from those expected from the redox mechanism. We discuss the mechanism for the photoinduced conduction change based on XAFS results and other experimental findings. 2. Experimental Section
Figure 1. (a) Molecular structure of DM and (b) crystal structure of Ag(DM)2 viewed along the stacking c axis. The positions of the Ag cations are indicated with “Ag”.
various photoproducts with different conductivities are possible. This redox mechanism is frequently used to explain the photoinduced conduction changes in Ag(DM)2. There are several Ag(DM)2 photoproducts, known as β, γ, δ, and ε, which are classified according to their appearance, chemical composition, and crystal structure.9 The most promising photoproducts for device application are β and γ, which have the same chemical composition as R and retain the original stoichiometry and the crystal shape. The β product is semiconductive with an identical appearance to R and is further classified into two different photoproducts, β1 and β2.18 The formation of these photoproducts is dependent on the duration of the UV-vis irradiation; less than 40 h irradiation produces β1, whereas longer irradiation produces β2. The β1 form has the same lattice constants as R but contains more lattice defects, whereas powder X-ray diffraction (XRD) patterns suggest that the β2 product is a mixture of R, β1, and a newly formed crystalline structure. The structure of β2 is complex and is presently under investigation, in addition to its chemical and physical properties. We concentrated only on the β1 form in this investigation. The insulating γ form is another promising photoproduct. It retains the chemical formula of R and preserves the DM molecular structure, but its structure is amorphous, unlike the R form, and it is a thermodynamically more stable state than R.19 Moreover, γ is diamagnetic, in contrast to the Pauli paramagnetism of R, and is transparent, which implies that the unpaired electrons are completely absent. The mechanism for the transformation from R to γ and the pairing of previously unpaired electrons remains to be clarified. In this paper, we investigated β1 and γ by the X-ray absorption fine structure (XAFS) technique in order to verify the redox mechanism and clarify the detailed geometry of γ. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were employed as powerful tools for elucidation of the electronic state and the local structure of an X-ray-absorbing atom. Information regard-
Ag(DM)2 was synthesized using a previously reported method.20 A 200 W Hg/Xe lamp was used to produce β1 and γ from the R form with UV-vis light (200-1100 nm).9,18,19 The two products were identified by their electrical, structural, spectroscopic, and magnetic properties. Powder XRD confirmed that the β1 photoproduct had the same crystal lattice parameters as those of R and that there was no formation of β2. Ag L3-edge XANES experiments were performed using the 11B beamline at the Photon Factory (PF), and spectra were recorded in the total electron yield mode and the Auger electron yield mode. In the latter case, the Ag LMM Auger electrons, with a kinetic energy around 2560 eV, were selected using a hemispherical energy analyzer. The spectra were obtained by averaging the Auger peak intensity in the kinetic energy range from 2552 to 2562 eV. The X-rays from the 2.5 GeV storage ring were monochromatized using a Ge(111) double crystal. X-ray spectra were then measured under ultra-high-vacuum (UHV) conditions at RT. To obtain reliable spectra, finely ground samples were fixed on conducting carbon tape. The spectra did not change even after overnight X-ray irradiation, which indicates that there was no irradiation damage. For the Ag L3-edge XANES analysis, the background absorption in the pre-edge region was removed by linear extrapolation. In the post-edge region, a smooth background absorption was obtained as a linear function from a least-squares fit. Normalization of the edge height was carried out by setting the value of the smooth background absorption at 3455 eV equal to 1. Ag K-edge EXAFS experiments were performed using the NW10A beamline at the PF-Advanced Ring (PF-AR), and the spectra of powder samples were recorded in the transmission mode at 15 K. X-rays from the 6.5 GeV PF-AR storage ring were monochromatized using a Si(311) double crystal. The Ag K-edge EXAFS oscillation was extracted using a spline smoothing method, followed by normalization to the edge height. The k3-weighted EXAFS oscillations were then Fourier transformed into R space. Structural parameters were obtained by curvefitting analyses using empirically derived phase shift and amplitude functions.21 3. Results 3.1. Ag L3-Edge XANES Spectra in the Total Electron Yield Mode. Figure 2 shows Ag L3-edge XANES spectra for R, β1, γ, and Ag metal foil. The R spectrum has a clear edge peak due to the Ag-DM bond. On the other hand, the Ag metal foil spectrum does not show any edge peaks. The edge peak for Ag(DM)2 is mainly attributed to the dipole transition from a Ag 2p orbital to a Ag 4d-5s hybridized empty state. The local symmetry around the Ag cation in Ag(DM)2 is D2d, which permits the hybridization of Ag 4d and 5s orbitals. On the other hand, for zero-valent Ag metal, the edge peak intensity is reduced because one-half of the 4d-5s hybridized state is filled by a 5s electron. Both β1 and γ spectra exhibit edge peaks comparable to that of R, which indicates that these photoproducts retain a Ag cationic state. The edge peak shapes for R and β1 are almost the same: that is, the intensity and peak maximum energy are
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Figure 2. Ag L3-edge XANES spectra of (a) Ag metal foil, (b) R-Ag(DM)2, and photoproducts (c) β1 and (d) γ in the total electron yield mode.
identical within the experimental error. According to previous Raman studies, the DM radical anions in β1 are oxidized to an average formal charge of -0.44 per DM unit.8,18,22 If this is the case, the formal charge of the Ag cation should be approximately +0.88. However, the XANES peak shape of β1 is identical to that of R, which indicates that little photoreduction of Ag cations occurs after UV-vis irradiation of R. This difference is because the XANES spectrum is quite sensitive to the electronic state of Ag and at least a 10% change in the electronic state of Ag can be detected. On the other hand, the Raman spectra indirectly estimates the formal charge based on the vibrational frequency of the C-N and C-C bonds in the DM radical anion.15,23 The XANES results suggest that such a relation between Raman shift and formal charge cannot be applied to β1. The γ spectrum also clearly exhibits the edge peak, which indicates that the chemical state of Ag is still the cationic state in γ. The fine structures of the post-edge region of the γ XANES spectrum are different from those of R, and the edge peak intensity is slightly higher, which suggests that the local structure around Ag cations in γ is different from that of R. 3.2. Ag L3-Edge XANES Spectra in the Ag LMM Auger Electron Yield Mode. It is possible that only Ag cations in the surface region are being reduced, which causes the photoinduced change in conduction. We investigated this possibility by measuring Ag L3-edge XANES spectra in the Auger electron yield mode, which is surface-sensitive due to its short mean free path (2-3 nm) and probes the surface chemical states, in contrast to the total electron yield mode, which monitors bulk Ag states. Figure 3 compares the Ag L3-edge XANES spectra acquired using both modes. All the spectra exhibit the same fine structures in either mode; the metal foil spectra show no edge peaks, and the R, β1, and γ spectra clearly show an edge peak in each spectrum. These results suggest that the surface chemical state of Ag at the surface and in the bulk is cationic in all photoproducts so that photoreduction does not occur, even in the surface region. Overall, the formal charge of Ag cations in β1 and γ is monovalent, as that in R. This contradicts the previously proposed redox photoreduction mechanism for the photoinduced changes in conduction observed for Ag(DM)2. Therefore, it is unclear why β1 is semiconductive. To evaluate this, the local structure around the Ag cations of β1 was investigated using Ag K-edge EXAFS. 3.3. Ag K-Edge EXAFS Spectra. Figure 4 shows Ag K-edge EXAFS Fourier transforms (FT) of R, β1, and γ. The
Miyamoto et al.
Figure 3. Ag L3-edge XANES spectra of (a) Ag metal foil, (b) R-Ag(DM)2, and photoproducts (c) β1 and (d) γ in the Ag LMM Auger electron yield mode (thick line) and the total electron yield mode (thin line).
Figure 4. Ag K-edge EXAFS Fourier transforms for (a) R, (b) β1, and (c) γ forms of Ag(DM)2.
spectra were recorded at 15 K in order to reduce thermal fluctuations. The first peak of the R spectrum (a, Figure 4) around 0.18 nm, the second peak around 0.27 nm, and the third peak around 0.41 nm correspond to the N, C, and N atoms of the N-C-N functional groups in the DM molecule, respectively. No Ag-Ag contribution from the nearest-neighbor Ag cations appears in the EXAFS FT spectra. The peak intensities of the second and third peaks are large due to a shadow effect resulting from the collinear arrangement of Ag-N-C-N atoms (164°).24 The β1 spectrum (b, Figure 4) also has three clear peaks, which are assigned as the first (N), second (C), and third (N) coordination shell atoms. The shape of the first peak is slightly different for β1 from that of R. A one-shell curve-fitting analysis for the first peak of the β1 spectrum using phase shift and amplitude functions derived from the R structure indicates that the coordination number is 3.2 (see Table 1). The second peaks of the FT for R and β1 are at the same distance and of similar intensity, which indicates that the Ag-(N)-C distance and coordination number are the same in both forms. However, the shapes of the third peaks in these two FTs are slightly different, and the peak intensity for β1 is smaller. These differences will be considered later.
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TABLE 1: One-Shell Curve-Fitting Results for β1 and γ, Assuming r Structurea photoproduct
coordination number
bond distance/nm
β1 γ
3.2 ( 0.2 2.2 ( 0.12
0.225 ( 0.006 0.215 ( 0.006
a Phase shift and backscattering amplitude functions are derived from the R assumptions: coordination number, 4; bond distance between Ag cation and N atom, 0.228 nm.
The γ spectrum (c, Figure 4) is much different from that of R. The first peak is much less intense, and the peak position has shifted toward a shorter bond distance. Moreover, the second peak almost disappears. These features suggest that the Ag-N bond has become shorter in γ and that the Ag-N-C bond angle has changed dramatically from the collinear arrangement in the R form. A one-shell curve-fitting analysis of the γ FT data is given in Table 1. The coordination number decreases to 2.2 and the bond distance is 0.215 nm, which indicates the cleavage of Ag-N bonds. Powder XRD measurements confirmed that γ is amorphous; therefore, the Ag cations in γ are surrounded by DM radical anions with a more distorted structure. 4. Discussion 4.1. Details of β1 Structure and Its Conduction Mechanism. The first peak of the β1 FT (ca. 0.18 nm) is broader than that of R (Figure 4). We have proposed that this peak broadening indicates the presence of another species with a slightly different bond length from that of R. The known features of β1 are summarized, and the structure of the new species is discussed. (1) β1 is a crystalline material with no amorphous content and a lattice structure identical to that of R.18 (2) Magnetic susceptibility measurements indicate the presence of a small amount (∼3%) of local spin in β1, which implies the generation of defects.25 (3) Elemental analysis indicates that β1 has the same stoichiometry as that of R, that is, of Ag(DM)2.9 (4) Raman and nuclear magnetic resonance (NMR) results indicate that the DM molecules maintain their molecular structure with only a slight change of their atomic positions. (5) The Knight shift of the NMR spectra confirms the presence of conduction electrons in most DM columns.18 (6) The XANES work reported here shows that almost all the Ag atoms in β1 remain in a monovalent state. (7) The EXAFS work reported here reveals that the Ag-N bond in β1 is strongly modified from that in R, but the Ag-(N)-C bond distance and coordination number remain unchanged. Taking all of these observations into account, there are three possible structural changes associated with the formation of β1: (A) The entire material alters its structure uniformly. (B) Defects are induced in R by UV-vis irradiation. (C) A new structure is partly created, which maintains a similar local structure and local symmetry around the Ag cations. The first possibility (A) can be rejected because the Ag cations are in a high local symmetry site with the S4 axis in the crystal structure, which does not explain the broadening of the first FT peak. The second possibility (B) is also unlikely because the number of defects observed from magnetic susceptibility measurements is as small as 3%, which is too small to be detected by EXAFS. Therefore, the third possibility (C) is concluded in that β1 is a mixture of R and a new structure belonging to the same space group as R. The new structure is
TABLE 2: Two-Shell Curve-Fitting Result for β1, Assuming r and r′ Structures structure R R′
fraction
bond distance/nm
0.83 ( 0.18 0.17 ( 0.005
0.228 (fixed) 0.219 ( 0.005
denoted as R′, which should have a different, but unique, Ag-N bond length from that of R. Because β1 is a mixture, then the EXAFS oscillations are the superposition of the EXAFS oscillations of R and R′. Accordingly, a two-shell curve-fitting was performed for the first peak, in order to evaluate the structure of the R′ component of β1. The coordination number was fixed at 4 in the R′ structure and the structure parameters of R in the curve-fitting analysis because the Ag cations in R′ should be coordinated by four N atoms according to the prerequisite of the S4 site symmetry. The fraction of the two species and the bond distance of Ag-N in R′ were then used as adjustable parameters. The results are shown in Table 2. The R′ structure has a shorter bond distance than that of the R structure. Approximately 20% of the original R structure changes into the R′ structure in β1. Because powder XRD observations suggested no amorphous character, R′ is a crystalline phase. The Ag K-edge EXAFS result shows that the Ag-N bonds in R′ become shorter, while the bond distance of the C atom in the second shell in R′ stays almost equal to that in R. The third shell FT peak decreases because the Ag(-N-C)-N bond distance in R′ changes. This subtle structural change of the R′ phase may not have a significant effect on the material molecular properties and lattice structure. As a result, no substantive changes are observed in the NMR and Raman spectra of β1 compared to those of R. Because the Ag cations must maintain their original position in R′ to remain in the same space group as R, the DM molecules must change their positions. The crystal structure in Figure 1b infers that the DM radical anions can change their orientation from their original position to make all the Ag-NCN bonds shorter, as occurs in the R′ phase. Because the change in orientation of all the DM radical anions in the R′ phase along the c axis is the same, the relationship between the quinone rings of the DM molecules is maintained in the R′ phase and the π overlap along the c axis is thus retained. This modification is collective and coherent over the entire R′ phase and acts to stabilize the metastable R′ structure. At the boundaries between R and R′ phases where mismatch in the π orbitals occur, the local spins are trapped because such lattice defects act to trap the conduction electrons, especially in 1D conductors.26-32 Therefore, a 3% local spin is observed in the magnetic susceptibility measurement. An activation energy is required when the electrons surmount the boundaries between two conducting R and R′ phases, and therefore, β1 becomes a semiconductive phase. 4.2. Details of γ Structure. The Ag K-edge EXAFS data for the γ phase shows that the first coordination peak appears at a shorter distance compared with that of R and β1 (ca. 0.18 nm in R and β1). The decrease in the coordination number indicates that the Ag-DM bond is cleaved. The disappearance of the second peak (ca. 0.27 nm in R and β1) is due to the destruction of the collinear arrangement of Ag-N-C bonds. The local environment around the Ag cations is dramatically changed in the γ form, even though the Ag(DM)2 composition ratio is maintained. The R structure should be stable because R is produced spontaneously by contacting Ag metal and DM molecules in the presence of organic solvents. Therefore, cleavage of the Ag-DM bonds should be difficult. A previous
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study reported that the γ product was also produced by heat treatment at ca. 428 K.19 Interestingly, the transformation from R (crystalline phase) to γ (amorphous phase) is exothermic (∆H ) -126.8 kJ mol-1), which indicates that γ is a more thermodynamically stable state than R. The change from R to γ is driven by a large internal energy gain, which exceeds that required to cleave the Ag-DM bond. After realizing this situation, we considered that polymerization or oligomerization of DM radical anions occurs after irradiation. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra measurements showed complex mass peaks19 and supported the formation of polymers or oligomers. The DM radical anions react with neighboring DM radical anions to form covalent bonds with the loss of unpaired electrons. The loss of unpaired electrons makes γ diamagnetic, transparent, and insulating. At the same time, the π-conduction band is destroyed with the formation of the amorphous phase. The bond formation between DM radical anions involves breaking the Ag-DM bond. Amorphous DM polymers and oligomers trap Ag cations with bent Ag-N(-CN-) bonding, such as that in the matrixisolated Ag species.33-36 5. Conclusion The Ag(DM)2 charge-transfer salt and its β1 and γ photoproducts were characterized using XAFS in order to reveal the mechanisms for the changes in conduction. The β1 product is a mixture of R and R′ phases. The atomic arrangement around the Ag cations in the R′ phase is only slightly modified from that of the original R material and occurs in a collective and coherent manner. The boundaries between the R and R′ phases cause β1 to be semiconductive. As for γ, the DM radical anions form covalent bonds among themselves, which leads to the formation of DM polymers or oligomers. This work provides a molecular-level description of the mechanism for conduction change in Ag(DM)2, which will contribute to the development of lithography-based electronic devices fabricated from a single material. Acknowledgment. XANES and EXAFS experiments were carried out under the approval of the PF Program Advisory Committee (PAC Nos. 2004G062 and 2006G059). T.M. wishes to acknowledge the financial support of the Japan Society for the Promotion of Science (JSPS) through a Research Fellowship for Young Scientists and a Grant-in-Aid for JSPS Fellows. Supporting Information Available: Figure A shows Ag K-edge EXAFS oscillations for (a) R-Ag(DM)2 and photoproducts (b) β1 and (c) γ recorded at 15 K. This material is available free of charge via the Internet at http://pubs.acs.org.
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