Air-Stable Thin Films with High and Anisotropic Electrical Conductivities Composed of a Carbon-Centered Neutral π-Radical.

Air-stable thin films (50-720 nm thickness) composed of a carbon-centered neutral π-radical with high and anisotropic electrical conductivities were fabricated by vapor deposition of 4,8,12-trioxotriangulene (TOT). The thin films were air-stable over 15 months and were the aggregate of TOT microcrystals, in which a one-dimensional π-stacking column was formed through the strong singly occupied molecular orbital (SOMO)-SOMO interaction with two-electron-multicenter bond among the spin-delocalized π-planes. The orientations of the one-dimensional column of TOT were changed depending on the deposition rate and substrates, where face-on-oriented thin films were epitaxially grown on the graphite 0001 surface, and edge-on-oriented thin films were grown on glass, SiO2, and indium tin oxide substrates under a high-deposition rate condition. The films showed high electrical conductivities of 2.5 × 10-2 and 5.9 × 10-5 S cm-1 along and perpendicular to the π-stacking column, respectively, for an edge-on oriented thin film.

Introduction

There have recently been intense challenges on the application of organic neutral radicals in various fields such as spin memories,[1,2] electrical conductors,[3,4] and thermal sensors[5] because a molecule having an unpaired electron works under a unique operating principle based on electronic spin.[6−8] There are several successful reports on thin-film fabrications, one of the most essential and general methods for the application to electronic devices, based on open-shell molecules such as nitroxide radicals,[9−13] cyclopentadienyl radical,[14] donor–acceptor type triphenylmethyl radicals,[15] metal radical complexes,[16] Blatter-type radicals,[12,17] singlet biradicaloids,[18,19] dithiadiazolyl radicals,[20] and bis(dithiadiazolyl) diradicals.[21,22] Neutral π-radicals, where the unpaired electron is included in a π-conjugated system, are a good candidate for thin-film fabrication because the delocalization of electronic spin increases thermodynamic stability.[23] For example, vapor-deposited thin films of bis(dithiadiazolyl) diradical showed photoconductivity,[22] and vapor-deposited thin films of phenalenyl-based singlet-biradicaloid exhibited ambipolar field-effect transistor (FET) properties.[18] Both radicals achieved one-dimensional (1D) structures by strong intermolecular spin–spin interactions, and their thin films showed electron-transport abilities through the 1D structures. In general, organic semiconductors are anisotropic, and electron transport parallel to the substrate is advantageous for FET, while that vertical to the substrate is advantageous for photovoltaics and light-emitting diodes.[24] The control of the molecular orientation within the thin film is, therefore, one of the most important subjects for the high-performance devices based on neutral π-radicals.

Although electrical conductivity is one of the representative transport properties, there are a few reports on thin-film electrical conductor based on neutral radicals. The bis(dithiadiazolyl) diradical, of which single crystal conductivity was very low (10–8 S cm–1) due to the strong π-dimerization within the π-stacking column,[25] gave a thin film with a room-temperature conductivity of 10–9 S cm–1.[22] It should be noted that the charge doping of the bis(dithiadiazolyl) diradical using iodine increases their conductivity in 10 orders in the crystalline state.[26] Recent reports on a nitroxide radical polymer thin film by the spin-coat and thermal annealing method exhibited a room-temperature conductivity of 0.28 S cm–1.[27] However as far as we know, the neutral radical thin film showing high and anisotropic electrical conductivity has not been reported.

We recently succeeded in the synthesis and isolation of 4,8,12-trioxotriangulene (TOT, 1) and its derivatives[23,28−34] as a new class of polycyclic carbon-centered organic neutral π-radicals with high air stability (Figure a).[35]TOT neutral radicals are treatable under an ambient condition in both solid and solution states even without steric protection groups, and no decomposition was found until 350 °C.[23] The origin of this high stability is spin delocalization over the 25π electronic system and electronic spin modulation by three oxygen atoms. TOT derivatives generally form face-to-face 1D π-stacking columns, namely, “π-stacked radical polymer”, in the solid state due to the strong intermolecular interaction of singly occupied molecular orbital (SOMO).[35] The orientation control of the 1D column in thin films is highly attractive because the 1D column plays an essential role for their electronic properties such as strong magnetic interaction,[23] high electrical conductivity (e.g., 2,6,10-tribromo-TOT 2: 1.8 × 10–3 S cm–1 in the crystalline state at room temperature)[30] and near-infrared photoabsorption.[29] Although neutral radical crystals with further higher electrical conductivities such as some zwitterionic bis(phenalenyl) boron complexes (0.3 S cm–1, at most)[36,37] and bis(dithiazolyl) radicals (4 × 10–2 S cm–1, at most)[38,39] have been reported, the electrical conductivities of TOT crystals are extremely high as a single component purely organic neutral radical. The pristine TOT 1 forms a π-dimer with a staggered overlapping through strong SOMO–SOMO interaction via the two-electron-multicenter bonds (pancake bonding),[40] and the π-dimers further stacks to construct a 1D column along the c-axis (Figure b).[23] In this report, we show the air-stable thin-film fabrication (50–720 nm thickness) of 1 by the conventional vacuum vapor deposition method as the first example of thin films of a condensed polycyclic carbon-centered neutral π-radical with high and anisotropic electrical conductivities (10–2 to 10–5 S cm–1). The molecular orientations (edge-on/face-on, Figure c) and morphologies of the thin films are changed by the deposition conditions, and we discuss the relationship between molecular orientations and electronic properties of the thin films.

Figure 1

(a) Chemical structure of 1, (b) crystal structure and 1D columnar structure of 1 determined by X-ray crystallography, (c) schematic images of molecular orientations in the thin films of 1. In (c), red-spaced plates indicate 1 molecules, and there is a small alternation in the plate-to-plate separation along the π-stacks.

Results and Discussion

Morphologies and Orientations in Thin Films on SiO2

Thin films of 1 were fabricated by the conventional vacuum vapor deposition method, and the conditions and properties of the films are summarized in Table S1 (50–720 nm thickness). The sublimation occurred over 150 °C at 0.2–6 mPa, and no residual and ash were left in the crucible. This result implies that 1 was stable in this sublimation condition and shows a good agreement with our previous experiments, where the bulk 1 neutral radical did not show any obvious decomposition even at 350 °C under air or N2 conditions.[23] The obtained thin films were highly air-stable, and no obvious decomposition was observed under air over 15 months and also annealing at 150 °C for 5 h in air (Figure S1). Morphologies of the thin films on SiO2 were observed by scanning electron microscopy (SEM) (Figure a–c). At a slow deposition rate (v = 0.2 Å s–1), two kinds of microcrystals, block- and wire-like shapes, were found (Figure a). When the deposition rate increased, the wire-like microcrystals gradually decreased, and the block-like microcrystal grains obviously became small (Figure b, v = 1.2 Å s–1). At v = 5.8 Å s–1, a uniform dense film consisting only of the block-like microcrystals was obtained (Figures c and S2). The drastic change of morphologies was also investigated by X-ray diffraction (XRD) analyses.

Figure 2

SEM images (a–d) and XRD spectra (e–h) of vapor-deposited thin films of 1. Thin films on SiO2 at the low deposition rate (v = 0.2 Å s–1) (a,e), at the middle deposition rate (v = 1.2 Å s–1) (b), at the high deposition rate (v = 5.8 Å s–1) (c,f), thin film on ITO at the high deposition rate (v = 6.3 Å s–1) (d,g), and (h) 1 powder. (i) Absorption spectra of thin films of 1 on the glass substrate. The thin films were set perpendicular to the optical axis.

The thin film fabricated at a low rate of v = 0.2 Å s–1 showed a set of peaks (marked peaks, 2θ = 27.5° and 30.9°) nearby the main peaks (Figure e) that were found for the powder sample (Figure h). The marked additional peaks would be afforded by the wire-like microcrystals because they were observed only in thin films of a low deposition rate (Figure a). The in-plane XRD pattern of the thin film on SiO2 at a high deposition rate (v = 5.8 Å s–1) showed a strong peak at 2θ = 27.0° (Figure f), which corresponds to the interlayer distance along the c-axis (d(102̅) = 3.26 Å, Figures S3 and S4). The out-of-plane XRD pattern, on the other hand, showed peaks at 2θ = 9.8, 16.8, 19.6, and 25.8°, which correspond to the periodic structures [d(21̅0) = 9.2, d(300) = 5.3, d(42̅0) = 4.2, and d(51̅0) = 3.5 Å, respectively] perpendicular to the π-stacking columns. These XRD spectra suggest that the microcrystals of 1 are aligned with the c-axis parallel to the SiO2 substrate and thus that the thin film has an edge-on orientation (Figure c). It has been reported that a π-molecule tends to stand perpendicular to a substrate giving an edge-on film, when the interaction between the π-molecule and the substrate is considerably weak in comparison with the interaction between π-molecules.[27] The strong SOMO–SOMO interaction between 1 molecules forming the π-stacking column would prevent the interaction between the π-electronic system of 1 and SiO2, and thus the edge-on orientation was constructed on the SiO2 substrate. In addition, the acid/base interaction of carbonyl and/or C–H on the TOT skeleton with a highly polar surface of SiO2 substrates may promote the formation of edge-on thin films. The diffraction peaks observed in the thin film at a high deposition rate (v = 5.8 Å s–1) were in good agreement with those of the single crystal (Figure S4). This indicates that the molecular arrangement in the thin film is identical to the crystalline state, and that the thin film is expected to exhibit a similar conductivity to that of the bulk sample.[23]

A thin film of 1 deposited on a glass substrate was reddish brown which is similar to the color of the powder sample (Figure S5). Absorption spectra with a perpendicular incidence for the thin films showed a broad and strong near-infrared peak around 1050 nm (Figure i). This result also suggests the edge-on orientation because this absorption band is characteristic for the 1D column of 1 and is anisotropically observed only in the direction perpendicular to the optical axis.[29] The absorption maxima showed a slight shift depending on the deposition rate. The spectrum of the high deposition rate (v = 5.8 Å s–1) film is similar to that of the pristine powder (λmax = 1038 and 1030 nm,[23] respectively). On the other hand, the peak top was shifted to 1096 nm at a low deposition rate (v = 0.2 Å s–1). The lower energy shift might be derived from minute differences in the π-stacking structure in the 1D column as shown in the wire-like microcrystals in the low-deposition rate film (Figure a).[23,29] These observations indicate that 1 formed an edge-on-oriented film (Figure c) on the SiO2 substrate under the high deposition rate (v = 5.8 Å s–1), where the molecules at the interface layer contact with the substrate at the molecular edge and the π-stacking columns were parallel to the SiO2 or glass substrates. It is expected that the electrical conductivity parallel to the substrate is much higher than that perpendicular to the substrate.

Electrical Conductivities of Thin Films

It is well known that the morphology and arrangement of an organic molecule in vapor-deposited thin films are highly sensitive to the substrates.[27,41] The vapor deposition of 1 on indium tin oxide (ITO) at a high deposition rate (v = 6.3 Å s–1) formed a thin film with a different morphology from that on the SiO2 substrate, where the film consisted of a larger size of microcrystals (Figure d). The in-plane XRD pattern showed a strong peak at 2θ = 27.0° similar to the films on the SiO2 substrate, and there was no peak in the out-of-plane measurement (Figure g), indicating that 1 formed another edge-on thin film on the ITO substrate. In an edge-on-oriented film, the 1D columns of 1 are parallel to the substrate (Figure c), the electron mobility parallel to the substrates would be advantageous than that perpendicular to the substrate.[39] The electrical conductivity of the thin film of 1 on the glass substrate at the high deposition rate (v = 6.3 Å s–1) was measured by the two-probe method (Figure a,b). The electrical conductivity parallel to the substrate was found to be 2.5 × 10–2 S cm–1 at the room temperature (Figure c). This very high electrical conductivity is similar to that of the single crystal of 0.32 S cm–1 at room temperature measured by the four-probe method as expected (Figure c). The slight decrease in the electrical conductivity in the thin film is caused by the randomness of the orientation of the 1D columns within the parallel direction to the substrate, and a higher conductivity is expected if the orientation of the 1D columns is aligned in this direction. The electrical conductivity of the thin-film became smaller with decreasing temperature because of the small semiconducting activation energy (82 meV), which is also comparable to that of the single crystal (90 meV). The similarity of the conducting properties of the thin film and single crystal implies that the effect of impurity during vapor deposition is negligible. It should be also noted that the thin-film electrical conductivity of 1 (2 × 10–5 S cm–1) is much higher than the thin films prepared from of single-component organic molecules such as bis(dithiadiazolyl) diradical (10–9 S cm–1)[22] and 2,5-bis-methylthio-7,7′,8,8′-tetracyanoquinodimethane (2 × 10–5 S cm–1),[42] and also comparable to the single-crystal conductivities of zwitterionic bis(phenalenyl) boron complexes (0.3 S cm–1)[36,37] and bis(dithiazolyl) radicals (4 × 10–2 S cm–1).[38,39] The high electrical conductivity of the thin film of 1 originates not only from the highly oriented π-stacking columns as the conducting pathway but also from the suppression of grain boundary resistance in the uniform dense thin film. On the other hand, the electrical conductivity vertical to the thin film, which is perpendicular to the π-stacked column (Figure b), was measured as 5.9 × 10–5 S cm–1. Although the anisotropy of electrical conductivities of these two films cannot be directly compared because the substrates were not the same, the electron mobility along the π-stacking column is significantly higher than that perpendicular to the column.

Figure 3

(a,b) Schematic images of the edge-on thin-film devices for two-probe electrical conductivity measurements. (a) Device for the measurement parallel to the substrate, where two comb-shaped gold electrodes were formed on the thin-film of 1 on the glass. (b) Device for the measurement perpendicular to the substrate, where a gold electrode was formed on the thin film of 1 on an ITO film. (c) Electrical conductivities (σ) of the edge-on films (d = 237 nm) and a single crystal of 1 as a function of the reciprocal temperature. Filled circle: conductivity parallel to the substrate of the thin film; open circle: conductivity of single-crystal of 1.

Thin Film on Graphite

Heteroepitaxy is often used to obtain highly ordered thin films.[43] Graphite has rich π-electrons on the 0001 surface of a hexagonal crystal system (a = b = 2.46 Å, γ = 120°) and is often used as the substrate for epitaxially face-on oriented ultrathin and/or monolayer films of polycyclic aromatic carbons.[41,44,45] The crystal system of 1 is trigonal (a = b = 18.2 Å, γ = 120°),[23] and the c-axis epitaxial growth with a face-on orientation is expected because of the acceptable misfit ({d(100)substrate – d(800)TOT}/d(100)substrate = 7.5%) in organic heteroepitaxial systems.[45] Furthermore, it has been reported that weak intermolecular interactions such as hydrogen bonds parallel to the π-molecular plane promote the face-on orientation in thin films.[46,47] In the crystal structure of 1, a 2D network is constructed by C–H···O hydrogen bonds parallel to the molecular plane (ab-plane)[23] and is expected to stabilize the face-on orientation. The vapor-deposited thin film of 1 on the graphite 0001 surface of highly oriented pyrolytic graphite (HOPG) (Figure a) and graphite sheets (Figures b–d) showed drastically different morphologies compared with those on the SiO2 and glass substrates. The cross-section SEM images of the thin film clearly showed that columnar microcrystals of 1 densely stood on the substrates to form sheer cliff-like morphologies (Figure c,d). The out-of-plane XRD pattern of the thin film on a well-peeled graphite sheet showed only a peak at 2θ = 27.0° that corresponds to the interlayer distance (d(102̅) = 3.26 Å) in the c-axis together with the peaks of graphite substrates (Figure e, top). On the other hand, the in-plane XRD pattern showed peaks that correspond to the periodic structures perpendicular to the 1D columns in the ab-plane, and the (102̅) peak at 2θ = 27.0° was not found (Figure e, bottom). These observations clearly indicate that the thin films on graphite substrates have an almost completely face-on orientation on the graphite 0001 surfaces. The drastic molecular orientation change between the films on SiO2 and graphite was also seen in the change of the color of each film. A vapor-deposited thin film on the single-layer graphene-SiO2 substrate was reddish brown, while that on the SiO2 substrate was greenish brown (Figure f). This is because the 1D column perpendicular to the optical axis selectively absorbs red to near-infrared light (edge-on film, 700–1300 nm, Figure g). The anisotropy of the electrical conductivity of the face-on film is expected to be exactly opposite to that of the edge-on film, and it is expected that the conductivity is higher in the vertical direction. However, because of the contact resistance between the thin film and the electrode as well as the difficulty in the deposition of a metal electrode on the thin film, we could not perform the accurate measurement of the electrical conductivity. The face-on orientation was kept up to the film thickness of 700 nm at least in the SEM observation regardless of the deposition rate (v = 0.1–3 Å s–1, Table S1), showing that the SOMO–SOMO interaction in the 1D π-stacking column was strong enough to set more than 1500 molecules.

Figure 4

SEM images of the surface (a,b) and cross-section (c,d) of thin films of 1 fabricated on the graphite 0001 surface. Substrates: (a) HOPG, (b,c) graphite sheet, and (d) graphite sheet peeled 24 times with scotch tape. (e) Out-of-plane and in-plane XRD spectra of the thin film of (d). (f,g) Photograph and absorption spectra of thin films fabricated on graphene/SiO2 and SiO2.

Conclusions

In conclusion, by utilizing the high stability of the TOT neutral radical, we successfully fabricated air-stable thin films of 1 by the conventional vapor-deposition method as the first thin film of the neutral π-radical having a fused polycyclic structure with high and anisotropic electrical conductivities (10–2 to 10–5 S cm–1). A large number of stable neutral radicals have been reported, however, those stable even in the vaporized state under high temperature and vacuum (0.2–6 mPa) conditions to give a vapor-deposited thin film are limited.[9−12,15,17−22] In the TOT thin films, 1D π-stacking columns were formed by the strong SOMO–SOMO interaction. The anisotropic orientation of the 1D columns depended on the substrates: edge-on-oriented thin films were obtained on glass, SiO2, and ITO under a high deposition rate, and face-on oriented thin films were achieved on the graphite 0001 surface. The electrical conductivity of the edge-on oriented thin films showed high anisotropy: the electrical conductivity parallel to the substrate was considerably higher than that vertical to the substrate (10–2 vs 10–5 S cm–1). It is worth noting that the neutral radical thin films showing anisotropic and high electrical conductivity have not been known before the TOT thin film of the present study. The present result suggests the potential for the TOT neutral radical as a molecular building block of organic electronic materials and may provide a new milestone in the material exploration in the development of organic electronic devices. Moreover, the 1D structure of π-radical aggregation and the resultant anisotropic electrical conductivity would be applied to novel applications based on radicals for such as Haddon’s hypothesis,[3] spintronics devices, and organic thin-film rechargeable lithium-ion batteries without conductive additives in the electrode. More superior electrically conductive thin films would be obtained by using mixed-valence compounds prepared by carrier doping of the neutral π-radical.[26,30]

Experimental Section

Materials

Neutral radical 1 was prepared as we had reported.[23] All of the substrates were commercially available materials. The nonalkali glass and ITO/glass substrates were cleaned in an ultrasonic bath for 10 min in Shibata clean A (Shibata Scientific Tech. Ltd.), pure water, and 2-propanol. The substrates dried in an oven were then exposed to a UV-ozone atmosphere for 30 min. HOPG was freshly cleaved with the Scotch tape. Graphite sheet (GRAPHINITY, KANEKA Corp.) was used without pretreatment (SEM measurement) or peeled 24 times with the Scotch tape (SEM and XRD measurement). Single-layer graphene–SiO2 (Graphene Platform Corp.) was used as purchased.

Vapor Deposition

The thin films of 1 were fabricated on the substrates by vacuum evaporation under a pressure of 0.2–6 mPa, using vapor evaporation chambers, ULVAC VPC-260 or Kenix KXV-250. The substrates were horizontally fixed above the evaporation source in a crucible (d = 30 or 90 mm), and the crucible was gradually heated from room temperature to 220–270 °C, and the sublimation occurred over 150 °C. Almost all substrate 1 mounted in the crucible evaporated under the deposition conditions. The deposition was monitored by the measurement of the thickness by a quartz crystal resonator in the chamber. The thickness of the resulting film was measured using the cross-section SEM image or stylus profilometry (Veeco Dektak 150) because of the poor sensitivity of the quartz crystal resonator especially in the case of d = 30 mm. The averaged deposition rate was obtained by dividing the film thickness by the deposition time. The deposition conditions and thickness are summarized in Table S1. The Au/1/ITO and Au/1/glass cells were fabricated by the following method: Au electrodes were evaporated through shadow masks in a vacuum with a pressure of ca. 2 mPa onto the thin films of 1 deposited on an ITO electrode or a glass substrate. The cells were annealed at 150 °C for 5 h before electrical measurement.

Measurements

Electronic spectra of KBr pellets or thin films on glass substrates were measured on a UV/visible/NIR scanning spectrophotometer (HITACHI U-4000). Morphology of the thin films was observed on a high-resolution SEM (Hitachi High-Technologies FE-SEM SU6600). The XRD measurements were performed by a diffractometer (Rigaku SmartLab). The temperature-dependent electrical conductivities of the edge-on film parallel and perpendicular to the substrate were measured using the direct current (dc) two-probe method. As for the measurement parallel to the substrate, the two-probe method using comb-shaped electrodes was performed because of very high resistance. The gold comb-shaped electrodes were formed by vacuum evaporation of gold on the film of 1 with a gap of 0.25 mm (top contact, Figure a). In the case of the measurement perpendicular to the substrate, the thinness of the film prevented a measurement with the four-probe method, and we measured in the two-probe method, where the films of 1 were formed on the ITO electrode, and then, gold was evaporated on the film of 1. The cells were attached onto the thermal stage (As-One HI-1000), and electrical measurement data parallel and perpendicular to the substrate were obtained using a picoammeter/voltage source (Keithley 6487) and dc voltage current source (ADCMT 6243), respectively. dc electrical conductivity of the single crystal of 1 along the π-stacking direction was measured by using a Keithley 2001 multimeter combined with the standard four-probe technique. Four gold wires of 10 μm diameter were attached to a single crystal of 1 with carbon paint (Jeol Dotite Paint XC-12) using gold wires of 10 μm diameter using carbon paste.