thermal and optical modulation of the carrier mobility in...

Thermal and Optical Modulation of the Carrier Mobility in OTFTsBased on an Azo-anthracene Liquid Crystal Organic SemiconductorYantong Chen,† Chao Li,‡ Xiuru Xu,† Ming Liu,† Yaowu He,† Imran Murtaza,§,∥ Dongwei Zhang,†

Chao Yao,† Yongfeng Wang,*,‡ and Hong Meng*,†,§

†School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, China‡Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China§Key Laboratory for Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic InnovationCenter for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816, China∥Department of Physics, International Islamic University, Islamabad 44000, Pakistan

*S Supporting Information

ABSTRACT: One of the most striking features of organicsemiconductors compared with their corresponding inorganiccounterparts is their molecular diversity. The major challengein organic semiconductor material technology is creatingmolecular structural motifs to develop multifunctionalmaterials in order to achieve the desired functionalities yetto optimize the specific device performance. Azo-compounds,because of their special photoresponsive property, haveattracted extensive interest in photonic and optoelectronicapplications; if incorporated wisely in the organic semi-conductor groups, they can be innovatively utilized in advanced smart electronic applications, where thermal and photomodulation is applied to tune the electronic properties. On the basis of this aspiration, a novel azo-functionalized liquid crystalsemiconductor material, (E)-1-(4-(anthracen-2-yl)phenyl)-2-(4-(decyloxy)phenyl)diazene (APDPD), is designed and synthe-sized for application in organic thin-film transistors (OTFTs). The UV−vis spectra of APDPD exhibit reversiblephotoisomerizaton upon photoexcitation, and the thin films of APDPD show a long-range orientational order based on itsliquid crystal phase. The performance of OTFTs based on this material as well as the effects of thermal treatment and UV-irradiation on mobility are investigated. The molecular structure, stability of the material, and morphology of the thin films arecharacterized by thermal gravimetric analysis (TGA), polarizing optical microscopy (POM), (differential scanning calorimetry(DSC), UV−vis spectroscopy, atomic force microscopy (AFM), and scanning tunneling microscopy (STM). This study revealsthat our new material has the potential to be applied in optical sensors, memories, logic circuits, and functional switches.

KEYWORDS: photoresponsive, azo-compound, liquid crystal, anthracene, organic thin-film transistors

1. INTRODUCTION

Owing to the wide applications of organic thin-film transistors(OTFTs) in integrated circuits, active matrix displays, flexibledisplays, and sensors, organic semiconductors have attractedmuch attention over the past two decades.1,2 Compared withinorganic thin-film transistors based on Si, OTFTs possessmany appealing advantages such as easy and diversemodifications of the organic molecular structures, simple andlow-cost fabrication approaches, and flexibility.3,4 As such,various synthetic strategy methods and structure−propertystudies have been devoted to design organic functionalsemiconductor materials with the goal to achieve high carriermobility, good environmental stability, and optimized specificdevice performance.5−7 Among numerous semiconductormaterials, acenes have shown excellent performance becauseof their strong intermolecular overlap stemming from the stableand powerful π−π* conjugated system.8,9 However, thestability of this class of organic semiconductors is not as

good as expected, such as pentacene and its derivatives.10,11 Torealize commercial applications, the conjugated core moiety hasbeen shortened for lowering the highest occupied molecularorbital (HOMO) in order to overcome the instabilityproblem.12,13 Various anthracene derivatives have beeninvestigated and reported. In all cases, these anthracene-basedorganic semiconductors showed improved stability withrelatively good device performance.14,15 Particularly, a recentlyreported anthracene semiconductor material with a phenylgroup covalently attached to the anthracene core exhibitedextended π−π* conjugated properties and demonstratedremarkably high carrier mobility, >10 cm2 V−1 s−1.16

On the other hand, research efforts have been devoted to thesynthesis of new materials with tunable optical and electronic

Received: October 22, 2016Accepted: February 1, 2017Published: February 1, 2017

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© 2017 American Chemical Society 7305 DOI: 10.1021/acsami.6b13500ACS Appl. Mater. Interfaces 2017, 9, 7305−7314

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properties. Among these, integrating a photochromic system inan OTFT material or device is believed to be a smartcomponent that efficiently and reversibly interconvertsfunctions at different stages. For example, photochromicmolecules that can reversibly switch between two isomers inresponse to light have been studied to develop functionaltransistors.17 These photochromic molecules, however, aremostly applied as a dielectric layer or doped into thesemiconductor layer and are rarely used directly as the activelayer in OTFTs.17−19 A simple way to implement amultifunctional OTFT semiconductor material in a device isan attractive strategy. Azobenzene chromophore, a kind ofphotochromic molecule, has been demonstrated with out-standing photoisomerization features due to their reversibleconformational changes in response to photoexcitation.20,21

They possess wide applications including reversible phase-transfer nanoparticles, light-controlled switches, and photo-responsive self-assembly vesicles.22−24 Generally, azo-com-pounds always exist in the trans-form, which is thermodynami-cally stable under natural conditions and will convert to cis-form under UV-irradiation of proper wavelength. The inverseprocess, from cis-form to trans-form, emerges upon visible lightexposure or heat treatment.25 Previous studies have revealedthat OTFTs based on directly deposited azobenzene molecules,as active layer, show low mobility and poor stability.26 Weproposed the design and synthesis of a novel azo-functionalizedOTFT material by attaching the azobenzene unit to ananthracene core to balance the charge mobility and photo-chromic performance.Previous research has demonstrated that a highly ordered

liquid crystal phase can abet the organic semiconductormolecule to arrange in a better orientation so that the carriertransport ability is improved.27−29 Therefore, we introduce along alkoxy chain at the other side of the azo-compound, whichcontributes to the liquid crystal phase, to achieve better carriertransport ability with thermal modulation character. The hybridanthracene semiconductor named APDPD, containing a

thermal and photochromic azo group linked with long alkoxychain mesogens, is intended to take advantage of all functionalproperties to confer a distinct property. The creativecombination of photochromic, liquid crystal, and anthracenegroups has potential applications in optoelectronic devices,such as optical sensors, memories, logic circuits, and functionalswitches.

2. EXPERIMENTAL DETAILSMaterials. All the reagents and solvents purchased were used

without further purification except as otherwise mentioned. Thesynthetic routes are shown in Scheme 1. 2-Bromoanthracene (2), 2-anthraceneborate (3), 1-(decyloxy)-4-nitrobenzene (5), 4-(decyloxy)-aniline (6), and (E)-4-((4-(decyloxy)phenyl)diazenyl)phenol (7) wereprepared according to the literature procedures.21,30,31

Synthesis. Preparation of (E)-4-((4-(Decyloxy)phenyl)diazenyl)-phenyl Trifluoromethanesulfonate (8). A 250 mL flask containing(E)-4-((4-(decyloxy)phenyl)diazenyl)phenol (19.35 mmol) wasequipped with a magnetic bar and protected by N2. Then 100 mLof dry dichloromethane and 8.05 mL of triethylamine were added, andthe solution was cooled to −20 °C. After that, 4.84 mL of triflicanhydride was added by dripping slowly with a syringe. The reactionmixture was allowed to warm up to room temperature after stirring at−20 °C for 1.5 h. After reaction, 100 mL of dichloromethane wasadded to the mixture before it was washed with 100 mL of water and100 mL of brine, three times for both. The organic layer was driedover Na2SO4, and the solvent was removed by evaporation underreduced pressure. Then the crude product was purified through silicagel column chromatography (petroleum ether/dichloromethane = 4/1), and 7 g of pure material was obtained as an orange solid with ayield of 75%. 1H NMR (300 MHz, CDCl3) δ 7.99−7.87 (m, 4H), 7.40(d, J = 9.0 Hz, 2H), 7.01 (d, J = 9.0 Hz, 2H), 4.05 (t, J = 6.6 Hz, 2H),1.88−1.77 (m, 2H), 1.51−1.23 (m, 14H), 0.88 (t, J = 6.7 Hz, 3H). 13CNMR (75 MHz, CDCl3) δ 162.50 (s), 152.13 (s), 150.36 (s), 146.73(s), 125.29 (s), 124.38 (s), 122.17 (s), 118.90 (q, J = 320.9 Hz),114.96 (s), 68.60 (s), 32.05 (s), 29.70 (s), 29.52 (s), 29.47 (s), 29.31(s), 26.15 (s), 22.83 (s), 14.26 (s) (1H NMR spectrum and 13C NMRspectrum are shown in Figures S1 and S2).

Preparation of (E)-1-(4-(Anthracen-2-yl)phenyl)-2-(4-(decyloxy)-phenyl)diazene (9). A 100 mL pressure bottle containing (E)-4-((4-

Scheme 1. Synthetic Routes of APDPD

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b13500ACS Appl. Mater. Interfaces 2017, 9, 7305−7314

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(decyloxy)phenyl)diazenyl)phenyl trifluoromethanesulfonate (4mmol), 2-anthraceneborate (4.4 mmol), and Pd(PPh3)4 (0.2 mmol)was filled with N2 for 15 min, and then was added 8 mL of ethanol, 32mL of toluene, and 8 mL of 2 M K2CO3 aqueous solution sequentiallyunder N2. After that the mixed solution was set to 90 °C and keptheating overnight. When the reaction was finished, the whole systemwas filtered and the filtrate was collected to be washed with water,ethanol, and acetone successively and cautiously. The dried crudeproduct was further purified by sublimation through a 5-zone furnacewith a vacuum degree of 1 × 10−3 Pa and heating temperatures of 260,220, 180, 140, and 80 °C, respectively (Figure S3). Finally, 1.4 g ofpure material was obtained as an orange solid with a yield of 70%.Because of the low solubility of APDPD, we could not obtain its 1HNMR data; instead, we select the high-resolution mass spectrum,which is provided in Figure S4, and elemental analysis (calculated forC36H38N2O: C, 84.01; H, 7.44; N, 5.44. Measured: C, 84.12; H, 7.41;N, 5.34) to confirm the structure of APDPD.Device Fabrication. The OTFT devices with bottom-gate, top-

contact structures were fabricated by vacuum-deposition on the heavilydoped silicon wafers with predeposited 300 nm silicon dioxide layer.Before use, the silicon wafers with silicon dioxide coating were washedwith acetone, deionized water, and isopropanol in ultrasonic cleanerfor 15 min, successively. After that the silicon wafers were placed underUV-irradiation for 20 min. Then they were modified by octyltri-chlorosilane (OTS) in anhydrous toluene with a concentration of 0.1mol/L at 60 °C for 40 min in the glovebox, washed with anhydroustoluene three times immediately, and dried by N2 gun. Thesemiconductor material was then evaporated onto the OTS-modifiedSiO2/Si substrate under a pressure of 2.4 × 10

−4 Pa, and Au, as sourceand drain electrodes, was deposited on the semiconductor layerthrough a shadow mask. The mobility of the OTFT devices wascalculated by the following equation under the saturation regime,

μ= −I W L C V V( /2 ) ( )iD G T2

where ID represents the source-drain current, W and L represent thechannel width and length, respectively, μ represents the mobility of theOTFT device, Ci represents the capacitance of gate insulator per unitarea, and VG and VT represent the gate and threshold voltages,respectively.

3. RESULTS AND DISCUSSIONIn our research, a new semiconductor material, APDPD, withboth thermal and photochromic properties, is synthesized andinvestigated. The UV−vis spectra of APDPD in dichloro-methane show variations after frequent treatment with UV-irradiation (with a handheld UV lamp of 6 W and 254 nm),heating, and visible light irradiation. The reversible isomer-ization of trans−cis−trans can be observed in the UV−vis

spectra (Figure 1a). The intensity of the absorption peak at λmax= 365 nm decreases and the edge segments of the spectrumincrease slightly under UV-irradiation until a photostationarystate appears, representing the transformation from trans-formto cis-form. The reverse process emerges with the originalspectrum reverting to another photostationary state undervisible light irradiation or heat treatment, which represents thecis-form converting back into trans-form.25 Because of thecompact molecular packing, the isomerization process in thesolid state requires more time than that in the solution and isnot very obvious (Figure S5). In addition, the onset absorptionwavelengths are found to be 449 nm in solution and 459 nm inthin film on quartz (Figures 1b and S6). According to the UV−vis spectra of the thin film, the optical band gap is calculated tobe 2.70 eV. The photoluminescence (PL) spectra of thematerial (APDPD) in solution and as a thin film were obtained,but the results show that APDPD is not luminescent like otheranthracene-based semiconductors.Furthermore, we also studied the electrochemical character-

istics of APDPD by cyclic voltammetry. The highest occupiedmolecular orbital (HOMO) and lowest unoccupied molecularorbital (LUMO) levels of −5.98 and −3.22 eV are obtainedfrom the onset of oxidation and reduction potentials of 0.88and −1.88 V versus Fc/Fc+, respectively. Electrochemicalenergy gap of 2.76 eV can be calculated from the differencebetween LUMO and HOMO levels, which is very close to thevalue measured from UV−vis spectra of thin films. APDPDshows excellent air stability due to its deep HOMO level(−5.98 eV), but it does not match well with the Au electrode(work function 5.1 eV). Therefore, the carrier mobility of theOTFTs based on APDPD can be improved if the energy barrieris reduced by further modification in the device structure. Thefrontier molecular orbitals for APDPD were calculated bydensity functional theory (DFT) using B3LYP functionalanalysis and 6-311G(d,p) basis set with Gaussian 09 package(Figure 2a). Besides that, the scanning tunneling microscopy(STM) constant-height dI/dV mappings of both negative biasand positive bias operation represent the HOMO and LUMOlevels, which are consistent with the DFT calculations (Figure2b). It is obvious from both DFT calculations and dI/dVmappings of STM that the electron distribution mostly locatesin anthracene at the HOMO level, while it moves to theazobenzene group at the LUMO level. The detailed optical andelectrochemical properties are summarized in Table 1.

Figure 1. (a) UV−vis spectra of APDPD in dichloromethane (CH2Cl2) under repetitive UV irradiation for 1 min and then exposure to visible lightfor 130 s (black arrows represent variation tendency of UV-irradiation and red arrows represent variation tendency of visible light irradiation). (b)UV−vis spectra of APDPD thin films on quartz (black line) and solution in dichloromethane (CH2Cl2) (red line). (c) Cyclic voltammetry ofAPDPD thin films on the glassy carbon electrode (GCE) (Φ = 3 mm) in CH2Cl2/TBAPF6 (tetrabutylammonium hexafluorophosphate) solution.



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With the long alkoxy chain in the APDPD molecule, thematerial presents typical liquid crystal characteristics withdistinctive thermal properties and optical textures as shown inFigure 3. A decomposition temperature of 355 °C at 5% weightloss is given by the thermal gravimetric analysis (TGA) curve(Figure 3a), and the three cycles of differential scanningcalorimetry (DSC) curves give the consistency of fourendothermic and exothermic peaks with the same peak positionand intensity (Figure S7), which shows a high thermal stability.To observe the thermal properties clearly, one cycle of DSCcurve is shown in Figure 3b that depicts four phase transitions.The first peak observed at 285 °C (obtained from the peakpoint) indicates a transition from isotropic to nematic (N)phase (ΔH = 0.99 kJ/mol) during the cooling process. Figure3c-1 shows the polarizing optical microscopy (POM) image ofnematic phase with marble texture. The second peak at 240 °Cwith the smallest enthalpy (ΔH = 0.15 kJ/mol) indicates thetransition from nematic to smectic A (SmA) phase. A fan-shaped texture is observed, and the typical droplet texture onthe material edges of smectic A phase is also found as shown inFigures 3c-2 and S8. These two liquid crystal phases performwith good fluidity similar to the isotropic state. With furtherreduction in temperature, the material begins to turn into ahighly ordered smectic phase with the third peak at 221 °C(ΔH = 53.6 kJ/mol), which shows a mosaic texture (Figure 3c-3) and is named as smectic X (SmX) phase as it is yet to becharacterized. The last peak emerged at 140 °C with theenthalpy of 9.58 kJ/mol, which indicates the transition fromsmectic X to crystalline state. The texture changes are verysubtle, and only some fine microgrooves are observed that canbe seen in Figure 3c-4.To investigate the semiconductor properties of APDPD,

which contains an anthracene part and a long π−π* conjugatedsystem, the OTFT devices with bottom gate/top contactstructure were fabricated by vacuum-deposition. Figure 4 shows

the output and transfer characteristics of p-type OTFTs aftertwo different ways of post-treatment with the consideration ofthermal and photochromic properties. In a series of treatmentswith thermal annealing (annealing process before depositingelectrode), the magnitude of mobility is found to be dependenton annealing temperature in the following order: annealing at100 °C < annealing at room temperature (RT) < annealing at200 °C < annealing at 150 °C. And the mobilities of the lattertwo devices are nearly an order of magnitude higher than thedevices at RT. The annealing temperatures of 150 and 200 °Cwere adopted for the reason of being suitable for forming ahighly ordered liquid crystal phase (SmX) and thus presentingbetter orientation of the molecules, which facilitates betterdevice performance. The high mobility in the devices annealedat 200 °C indicates the stability of our OTFT devices.Meanwhile, another series of treatments with UV-irradiation(with an UV lamp of 6 W and 254 nm) for 1 h, preceded bythermal annealing, presents remarkable results. The mobilityand on/off current ratio of all the devices increased dramaticallyafter UV-irradiation as compared to the first series oftreatments with thermal annealing. The highest mobility ofthe semiconductor material is calculated to be 0.875 cm2 V−1

s−1, demonstrated by the devices being annealed at 150 °C andtreated with UV-irradiation. This value is almost 20 timeshigher than the mobility of the devices at RT (0.047 cm2 V−1

s−1). The high mobilities of the devices after UV-irradiation areattributed to the synergistic effects of photoisomerization andphotoinduced molecular arrangement. The intrinsic behavior ofazo-functionalized liquid crystal semiconductor prompts asuperior alignment resulting in higher mobility after UV-irradiation.33,34 The reversibility of the photosensitive propertyof the devices was also investigated and observed. After UV-irradiation the device mobility increased dramatically, and afterbeing exposed to the vis-irradiation it again dropped to thesame level (Figure S9).The threshold voltages of all the devices are high and may be

caused by the energy-level barrier, which limits the carrierinjection and mobility.35,36 Some of the output curves showdevice instability (Figure 4b1, b2, b4, and b6), which may beattributed to the traps induced at high voltage due to the highhumidity, because it occurs in the devices with disorderedmolecular arrangement (RT and 100 °C) and cracks (200°C).37,38 All the mobilities are calculated by the typical metal-oxide−semiconductor field-effect transistor (MOSFET) equa-tion in the saturation regime, and the electrical characteristics ofthe specific devices are summarized in Table 2.The APDPD thin films, with a thickness of ∼40 nm, were

deposited via vacuum-evaporation on OTS-modified SiO2/Sisubstrates. The morphology of the thermally treated andsubsequently UV-irradiated (with a handheld UV lamp of 6 Wand 254 nm) thin films analyzed by atomic force microscopy(AFM) elucidates the difference in the mobility to some extent,as shown in Figure 5. Small grains with terraces are found at the

Figure 2. (a) Frontier molecular orbitals for APDPD calculated byDFT (B3LYP/6-311G(d,p) level). (b) dI/dV mapping of scanningtunneling microscopy (STM) under constant-height mode.

Table 1. Optical and Electrochemical Properties of APDPD

UV−vis (nm)a CV (V)b CV (eV)c UV−vis (eV)d calculatione

λedgesol λedge

film Eonsetneg Eonset

pos ELUMO EHOMO Eg Egsol Eg

film ELUMO EHOMO Eg

APDPD 449 459 −1.88 0.88 −3.22 −5.98 2.76 2.76 2.70 −2.35 −5.42 3.07

aSolution and film absorption edge. bElectrochemical reduction and oxidation potential determined versus Fc/Fc+. cELUMO = −(E[onset,red vs Fc/Fc+] +5.1), EHOMO = −(E[onset,oxi vs Fc/Fc+] + 5.1),32 Eg = ELUMO − EHOMO. dOptical band gap estimated from the absorption edge of the solution and thinfilm. eCalculated by DFT (B3LYP/6-311G(d,p) level).



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bottom of the film, but some disordered and haphazard worm-shaped grains are also observed on the thin-film surface, whichmay slightly impair the charge transport at RT (Figure 5a1). Itcan be seen that the surface morphology changes obviouslyafter annealing at 100, 150, and 200 °C. After annealing at 100°C, the worm-shaped grains aggregate together to form biggerrodlike structures without terraces and possess a disorientedarrangement (Figure 5a2), which is unfavorable to themolecular stacking and results in a decreased device mobilitycompared with that of the devices at RT. When the annealingtemperature is raised up to 150 and 200 °C, the materialachieves highly ordered liquid crystal phase (SmX) so that themolecules are able to realign, and after cooling naturally toroom temperature they rearrange to form larger grain sizeswithout any worm-shaped grains, thus resulting in a smooth,plain, and uniform film that deserves a better charge-transportproperty (Figure 5a3 and a4). Although the thin films annealedat both 150 and 200 °C possess the same liquid crystal phase(SmX), some cracks are observed in the film with the rise in thetemperature at 200 °C, which leads to a bit lower mobilitycompared with that of the thin films annealed at 150 °C(Figure 5a4).The cross section curves and detailed values of steps at 150

and 200 °C are shown in Figure 5a5 and a6, respectively,whereas the thin films at RT and 100 °C do not have anyobvious steps. The height of the terraces is measured to be 6.5nm on average, which is approximately twice the molecularlength (3.53−3.55 nm) calculated with the help of Multwfn(version 3.3.9) (Figure 6a),39 which indicates the molecularpacking structure (Figure S10).27,40 The calculations formolecular structure were carried out by density functionaltheory (DFT) using hybrid B3LYP functional with 6-311G-(d,p) basis set for geometrical optimization and single pointenergy, and all the calculations were performed in Gaussian 09package. Moreover, because monolayer molecular packing canonly be observed in a small region on the thin film annealed at

150 °C, it may be concluded that molecules are mostlyarranged in bilayers (Figure S11). After UV-irradiation of all thethin films for 1 h, through in situ measurements it isinterestingly found that only the thin films without annealing(RT) are slightly changed, with the growth in size of superficialgrains along their width while keeping the same shape (Figure5a1 and b1). It can be seen that morphologies of the otherthree thin films are almost unchanged (Figure 5a2 and b2, a3and b3, and a4 and b4). The height of the terraces still remains∼6.6 nm after UV-irradiation of the thin films annealed at 150and 200 °C (Figure 5b5 and b6), and little differences in themorphology are not able to demonstrate the high performanceof corresponding OTFT devices after UV-irradiation.A comparison of the XRD patterns of the thin films at RT,

annealed at 100, 150, and 200 °C and UV-irradiated, is shownin Figure 6b. The XRD patterns demonstrate the crystallinityand orientation of the molecules exactly in accordance with themorphology of the thin films shown by AFM images and thedevice performance. For all five different treatment methods,we can see four diffraction peaks, but the significant differenceof the intensity can only be seen in the first peak, as comparedwith the XRD pattern of the thin films at RT. In detail, at theannealing temperature of 100 °C, the thermal energy justfacilitates the molecular grains to aggregate physically (as seenin the AFM images), and this process disturbs the moleculararrangement; thus, the intensities of all diffraction peaksbecome weaker. When the annealing temperature goes up to150 °C, the molecules attain a highly ordered liquid crystalphase (SmX) and realign to form an ordered orientation withhigh crystallinity; therefore, the first peak possesses dramaticallyhigh intensity. Although at 200 °C it is still in the liquid crystalphase (SmX), the relatively high temperature makes the filmslightly crack, affecting the ordered arrangement andcrystallinity of the molecules to some extent. The appearanceof a fifth peak upon annealing at 150 and 200 °C also indicatesthat the molecules are rearranged in a highly ordered alignment

Figure 3. Thermal properties and optical textures of APDPD. (a) Thermal gravity analysis (TGA), (b) differential scanning calorimetry analysis(DSC), and (c) polarizing optical microscope (POM) images during the cooling process.



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within their liquid crystal phase.29 The intensity of the peaks,especially the first peak, increases after UV-irradiation, whichdemonstrates the enhancement of crystallinity and molecularalignment.Scanning tunneling micrographs of self-assembled mono-

layers of azo-liquid crystal compound APDPD on gold surfacewere obtained to investigate its photoisomerization and subtlestructure. Upon deposition at low APDPD concentration, STM

observations directly show well-ordered molecular arrangementon Au(111). As illustrated in Figure 7a and b, fishbone-shapedmolecular features are mostly assembled at the elbow sites ofAu(111), which induce the same orientation of most of themolecular self-assembled structural units.41 The high-resolutionSTM image in Figure 7c reveals the clear structure of individualAPDPD molecules within the fishbone-shaped supramolecularstructures. The stripe comprised of three different linking

Figure 4. Output and transfer curves of OTFT devices based on APDPD semiconductor with different treatment methods: (a1−a4) transfer curvesof OTFTs at RT and after annealing at different temperatures, (b1−b4) corresponding output curves, (a5−c8) transfer curves of OTFTs afterpostannealing treatment with UV-irradiation, (b5−b8) corresponding output curves.



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structures is in good agreement with the structure of APDPD.The molecular length is measured to be ∼3.3 nm, which is veryclose to the value of 3.53−3.55 nm obtained by calculations.Figure 7d exhibits the STM manipulation process in which thelateral stripe was pulled out and put back to further confirmthat each stripe represents an APDPD molecule. The molecularlayer was treated with electron excitation instead of UV-irradiation to explore the isomerization from trans-form to cis-form for their same essence. However, the photoisomerizationfailed through the STM experiment on monomolecular layerbecause the molecule−surface coupling quenched the proc-

ess.42,43 The mechanism of quenching has been demonstratedas follows: (i) Electronic lifetime effects: the time of a wholemolecular conformational changing process is longer than thelifetime of the excited electron acting on the surface.43,44 (ii)Substrate-induced changes in optical absorption: the opticalabsorption is changed and then affects the photoswitchingprocess because of the hybridization between molecules andsubstrate.44 In addition to this, the dramatically increasedmobility of OTFT devices after UV-irradiation may beattributed to the synergistic effects of photoisomerization and

Table 2. OTFT Devices Performance of APDPD with Different Treatment Process

material annealing temp (°C) UV-irradiation (254 nm) mobilityavg (cm2 V−1 s−1) mobilitymax (cm

2 V−1 s−1) Vth (V) Ion/Ioff

RT 0.042 0.047 −52.0 2.0 × 104

100 0.019 0.024 −53.6 1.1 × 104

150 0.229 0.302 −52.1 1.8 × 105

200 0.115 0.179 −51.4 1.1 × 105

APDPD RT 1 h 0.285 0.359 −41.1 6.1 × 105

100 1 h 0.088 0.105 −42.5 2.4 × 105

150 1 h 0.746 0.875 −43.0 5.2 × 106

200 1 h 0.474 0.602 −45.8 1.1 × 106

Figure 5. Normal AFM images of APDPD thin films fabricated on the OTS-treated SiO2/Si substrate with two series: (a1−a4) at RT and thermalannealing at 100, 150, and 200 °C and (b1−b4) UV-irradiation at 254 nm for 1 h after thermal annealing. (a5, b5 and a6, b6) Cross section curvesaccording to the thin film (direction follows the red arrows).



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photoinduced molecular arrangement, when the quenching isreduced between the molecular layers.

4. CONCLUSIONIn summary, a novel azo-functionalized liquid crystal material,APDPD, which is endowed with photochromic, thermal, andsemiconductor characteristics altogether, was synthesized andcorresponding OTFTs were fabricated. In the thermalannealing treatment process, highly ordered liquid crystalphase (SmX) with good orientation of the molecules facilitatesbetter device performance. Compared with all the thermallytreated devices, carrier mobilities of thin-film transistorsachieved greatly improvement with the highest mobility of0.875 cm2 V−1 s−1 after UV-irradiation for 1 h, and it is mainlydue to the synergistic effect of photoisomerization andphotoinduced molecular arrangement. This type of multifunc-tional materials will provide reference for future researches torealize their potential applications in optoelectronic devices,such as optical sensors, memories, logic circuits, and functionalswitches.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b13500.

1H NMR spectrum and 13NMR spectrum, simplifiedgraph of the sublimation, high-resolution mass spectrum,UV−vis spectra, differential scanning calorimetry analysis(DSC) for three cycles, polarizing optical microscope(POM) images of smectic A phase, the reversibility ofphotosensitive property of the device, diagrammaticsketch of the molecular packing structure, AFM imagesof APDPD thin films annealed at 150 °C andcorresponding cross section curve (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected].*E-mail: [email protected] Meng: 0000-0001-5877-359XNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the Shenzhen PeacockProgram (KQTD2014062714543296), National Natural Sci-ence Foundation of China (51603003), Guangdong KeyResearch (2014B090914003 and 2015B090914002), Guang-

Figure 6. (a) Ball-and-stick model of APDPD calculated by Multwfn (version 3.3.9). (b) X-ray diffraction patterns of APDPD thin film with differenttreatment methods: RT (black line), 100 °C annealing (red line), 150 °C annealing (blue line), 200 °C annealing (green line), and UV irradiation at254 nm for 1 h of the thin films at RT (pink line).

Figure 7. STM constant-current images of APDPD molecules on Au(111). (a) 90 × 90 nm2, (b) 18 × 18 nm2, (c) 9 × 9 nm2 (the measured unit cellparameters for this fishbone-shaped structure are a = 1.0 ± 0.1 nm, b = 4.2 ± 0.1 nm, and α = 94 ± 1°). (d) STM manipulation images of APDPDmolecules on Au(111).



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http://pubs.acs.orghttp://pubs.acs.org/doi/abs/10.1021/acsami.6b13500http://pubs.acs.org/doi/suppl/10.1021/acsami.6b13500/suppl_file/am6b13500_si_001.pdfmailto:[email protected]:[email protected]://orcid.org/0000-0001-5877-359Xhttp://dx.doi.org/10.1021/acsami.6b13500

dong Talents Project, Shenzhen Science and TechnologyResearch Grant (JCYJ20160510144254604), National BasicResearch Program of China (973 Program, 2015CB856505),Natural Science Foundation of Guangdong Province(2014A030313800), and Guangdong Academician Workstation(2013B090400016).

■ REFERENCES(1) Tang, Q.; Tong, Y.; Hu, W.; Wan, Q.; Bjørnholm, T. Assembly ofNanoscale Organic Single-Crystal Cross-Wire Circuits. Adv. Mater.2009, 21, 4234−4237.(2) Forrest, S. R. The Path to Ubiquitous and Low-Cost OrganicElectronic Appliances on Plastic. Nature 2004, 428, 911−918.(3) Tseng, H. R.; Phan, H.; Luo, C.; Wang, M.; Perez, L. A.; Patel, S.N.; Ying, L.; Kramer, E. J.; Nguyen, T. Q.; Bazan, G. C.; Heeger, A. J.High-Mobility Field-Effect Transistors Fabricated with MacroscopicAligned Semiconducting Polymers. Adv. Mater. 2014, 26, 2993−2998.(4) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.;Kastler, M.; Facchetti, A. A High-Mobility Electron-TransportingPolymer for Printed Transistors. Nature 2009, 457, 679−686.(5) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.;Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet Printing of Single-CrystalFilms. Nature 2011, 475, 364−367.(6) Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S.C.; Chen, J.; Nordlund, D.; Toney, M. F.; Huang, J.; Bao, Z. Ultra-High Mobility Transparent Organic Thin Film Transistors Grown byAn Off-Centre Spin-Coating Method. Nat. Commun. 2014, 5, 3005.(7) Qiu, Y.; Hu, Y.; Dong, G.; Wang, L.; Xie, J.; Ma, Y. H2O Effect onthe Stability of Organic Thin-Film Field-Effect Transistors. Appl. Phys.Lett. 2003, 83, 1644.(8) Anthony, J. E. Functionalized Acenes and Heteroacenes forOrganic Electronics. Chem. Rev. 2006, 106, 5028−5048.(9) Ye, Q.; Chi, C. Recent Highlights and Perspectives on AceneBased Molecules and Materials. Chem. Mater. 2014, 26, 4046−4056.(10) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Semiconductingπ-Conjugated Systems in Field-Effect Transistors: A Material Odysseyof Organic Electronics. Chem. Rev. 2012, 112, 2208−2267.(11) Roberson, L. B.; Kowalik, J.; Tolbert, L. M.; Kloc, C.; Zeis, R.;Chi, X. L.; Fleming, R.; Wilkins, C. Pentacene Disproportionationduring Sublimation for Field-Effect Transistors. J. Am. Chem. Soc.2005, 127, 3069−3075.(12) Dong, H.; Wang, C.; Hu, W. High Performance OrganicSemiconductors for Field-Effect Transistors. Chem. Commun. 2010, 46,5211−5222.(13) Aleshin, A. N.; Lee, J. Y.; Chu, S. W.; Kim, J. S.; Park, Y. W.Mobility Studies of Field-Effect Transistor Structures Based onAnthracene Single Crystals. Appl. Phys. Lett. 2004, 84, 5383.(14) Yan, L.; Zhao, Y.; Yu, H.; Hu, Z.; He, Y.; Li, A.; Goto, O.; Yan,C.; Chen, T.; Chen, R.; Loo, Y.-L.; Perepichka, D. F.; Meng, H.;Huang, W. Influence of Heteroatoms on the Charge Mobility ofAnthracene Derivatives. J. Mater. Chem. C 2016, 4, 3517−3522.(15) Meng, H.; Sun, F. P.; Goldfinger, M. B.; Gao, F.; Londono, D.J.; Marshal, W. J.; Blackman, G. S.; Dobbs, K. D.; Keys, D. E. 2,6-Bis[2-(4-pentylphenyl)vinyl]anthracene: A Stable and High Charge MobilityOrganic Semiconductor with Densely Packed Crystal Structure. J. Am.Chem. Soc. 2006, 128, 9304−9305.(16) Liu, J.; Dong, H.; Wang, Z.; Ji, D.; Cheng, C.; Geng, H.; Zhang,H.; Zhen, Y.; Jiang, L.; Fu, H.; Bo, Z.; Chen, W.; Shuai, Z.; Hu, W.Thin film Field-Effect Transistors of 2, 6-Diphenyl anthracene (DPA).Chem. Commun. 2015, 51, 11777−11779.(17) Orgiu, E.; Samorì, P. 25th Anniversary Article: OrganicElectronics Marries Photochromism: Generation of MultifunctionalInterfaces, Materials, and Devices. Adv. Mater. 2014, 26, 1827−1845.(18) Lee, K. E.; Lee, J. U.; Seong, D. G.; Um, M. K.; Lee, W. HighlySensitive Ultraviolet Light Sensor Based on Photoactive Organic GateDielectrics with an Azobenzene Derivative. J. Phys. Chem. C 2016, 120,23172−23179.

(19) Gemayel, M. E.; Borjesson, K.; Herder, M.; Duong, D. T.;Hutchison, J. A.; Ruzie, C.; Schweicher, G.; Salleo, A.; Geerts, Y.;Hecht, S.; Orgiu, E.; Samorı, P. Optically Switchable Transistors bySimple Incorporation of Photochromic Systems into Small-MoleculeSemiconducting Matrices. Nat. Commun. 2015, 6, 6330.(20) Guo, S.; Matsukawa, K.; Miyata, T.; Okubo, T.; Kuroda, K.;Shimojima, A. Photoinduced Bending of Self-Assembled Azobenzene-Siloxane Hybrid. J. Am. Chem. Soc. 2015, 137, 15434−15440.(21) Kim, D. Y.; Lee, S. A.; Kim, H.; Min Kim, S.; Kim, N.; Jeong, K.U. An Azobenzene-Based Photochromic Liquid Crystalline Amphi-phile for a Remote-Controllable Light Shutter. Chem. Commun. 2015,51, 11080−11083.(22) Peng, L.; You, M.; Wu, C.; Han, D.; Öcşoy, I.; Chen, T.; Chen,Z.; Tan, W. Reversible Phase Transfer of Nanoparticles Based onPhotoswitchable Host−Guest Chemistry. ACS Nano 2014, 8, 2555−2561.(23) Mativetsky, J. M.; Pace, G.; Elbing, M.; Rampi, M. A.; Mayor,M.; Samorì, P. Azobenzenes as Light-Controlled Molecular ElectronicSwitches in Nanoscale Metal-Molecule-Metal Junctions. J. Am. Chem.Soc. 2008, 130, 9192−9193.(24) Liu, Y.; Yu, C.; Jin, H.; Jiang, B.; Zhu, X.; Zhou, Y.; Lu, Z.; Yan,D. A Supramolecular Janus Hyperbranched Polymer and ItsPhotoresponsive Self-Assembly of Vesicles with Narrow SizeDistribution. J. Am. Chem. Soc. 2013, 135, 4765−4770.(25) Song, H.; Jing, C.; Ma, W.; Xie, T.; Long, Y. T. ReversiblePhotoisomerization of Azobenzene Molecules on a Single GoldNanoparticle Surface. Chem. Commun. 2016, 52, 2984−2987.(26) Arlt, M.; Scheffler, A.; Suske, I.; Eschner, M.; Saragi, T. P.;Salbeck, J.; Fuhrmann-Lieker, T. Bipolar Redox Behaviour, Field-EffectMobility and Transistor Switching of the Low-Molecular Azo GlassAZOPD. Phys. Chem. Chem. Phys. 2010, 12, 13828−13834.(27) Iino, H.; Usui, T.; Hanna, J. Liquid Crystals for Organic Thin-Film Transistors. Nat. Commun. 2015, 6, 6828.(28) He, Y.; Sezen, M.; Zhang, D.; Li, A.; Yan, L.; Yu, H.; He, C.;Goto, O.; Loo, Y. L.; Meng, H. High Performance OTFTs FabricatedUsing a Calamitic Liquid Crystalline Material of 2-(4-Dodecyl phenyl)[1]benzothieno[3,2-b][1]benzothiophene. Adv. Electron. Mater. 2016,2, 1600179.(29) He, Y.; Xu, W.; Murtaza, I.; Zhang, D.; He, C.; Zhu, Y.; Meng,H. Molecular Phase Engineering of Organic Semiconductors Based ona [1]Benzothieno[3,2-b][1]benzothiophene Core. RSC Adv. 2016, 6,95149.(30) Bailey, R. K.; Blanchet, G. B.; Catron, J. W.; Chesterfield, R. J.;Gao, F.; Glicksman, H. D.; Goldfinger, M. B.; Jaycox, G. D.; Johnson,L. K.; Keusseyan, R. L.; Malajovich, I.; Meng, H.; Meth, J. S.; O’Neil,G.; Sharp, K. G.; Tassi, N. G.; Nunes, G. Thin Film TransistorComprising Novel Conductor and Dielectric Compositions. U.S.Patent 7,528,448[P], May 5, 2009.(31) Wąsik, R.; Wińska, P.; Poznan ́ski, J.; Shugar, D. Synthesis andPhysico-Chemical Properties in Aqueous Medium of All PossibleIsomeric Bromo Analogues of Benzo-1H-Triazole, Potential Inhibitorsof Protein Kinases. J. Phys. Chem. B 2012, 116, 7259−7268.(32) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C.Electrochemical Considerations for Determining Absolute FrontierOrbital Energy Levels of Conjugated Polymers for Solar CellApplications. Adv. Mater. 2011, 23, 2367−2371.(33) Kundu, S.; Lee, M. H.; Lee, S. H.; Kang, S. W. In SituHomeotropic Alignment of Nematic Liquid Crystals Based onPhotoisomerization of Azo-Dye, Physical Adsorption of Aggregates,and Consequent Topographical Modification. Adv. Mater. 2013, 25,3365−3370.(34) Xue, C.; Xiang, J.; Nemati, H.; Bisoyi, H. K.; Gutierrez-Cuevas,K.; Wang, L.; Gao, M.; Zhou, S.; Yang, D.; Lavrentovich, O. D.; Urbas,A.; Li, Q. Light-Driven Reversible Alignment Switching of LiquidCrystals Enabled by Azo Thiol Grafted Gold Nanoparticles.ChemPhysChem 2015, 16, 1852−1856.(35) Kumaki, D.; Umeda, T.; Suzuki, T.; Tokito, S. High-MobilityBottom-Contact Thin-Film Transistor Based on AnthraceneOligomer. Org. Electron. 2008, 9, 921−924.



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(36) Soeda, J.; Hirose, Y.; Yamagishi, M.; Nakao, A.; Uemura, T.;Nakayama, K.; Uno, M.; Nakazawa, Y.; Takimiya, K.; Takeya, J.Solution-Crystallized Organic Field-Effect Transistors with Charge-Acceptor Layers: High-Mobility and Low-Threshold-Voltage Oper-ation in Air. Adv. Mater. 2011, 23, 3309−3314.(37) He, C.; He, Y.; Li, A.; Zhang, D.; Meng, H. BlendingCrystalline/Liquid Crystalline Small Molecule Semiconductors: AStrategy Towards High Performance Organic Thin Film Transistors.Appl. Phys. Lett. 2016, 109, 143302.(38) Qu, M.; Li, H.; Liu, R.; Zhang, S. L.; Qiu, Z. J. Interaction ofBipolaron with the H2O/O2 Redox Couple Causes Current Hysteresisin Organic Thin-Film Transistors. Nat. Commun. 2014, 5, 3185.(39) Lu, T.; Chen, F. Multiwfn: A Multifunctional WavefunctionAnalyzer. J. Comput. Chem. 2012, 33, 580−592.(40) He, Y.; Sezen, M.; Zhang, D.; Li, A.; Yan, L.; Yu, H.; He, C.;Goto, O.; Loo, Y.-L.; Meng, H. High Performance OTFTs FabricatedUsing a Calamitic Liquid Crystalline Material of 2-(4-Dodecylphenyl)[1]benzothieno[3,2-b][1]benzothiophene. Adv. Electron. Mater. 2016,2, DOI: 10.1002/aelm.201670053.(41) Wang, Y.; Ge, X.; Schull, G.; Berndt, R.; Bornholdt, C.; Koehler,F.; Herges, R. Azo Supramolecules on Au(111) with Controlled Sizeand Shape. J. Am. Chem. Soc. 2008, 130, 4218−4219.(42) Wu, C.; Li, J.; Lee, D. Phase Transition in a Two-level-cavitySystem in an Ohmic Environment. Eur. J. Inorg. Chem. 2007, 2010,1763−1766.(43) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser,F.; Harvey, J. H.; Strubbe, D. A.; Frechet, J. M.; Trauner, D.; Louie, S.G.; Crommie, M. F. Reversible Photomechanical Switching ofIndividual Engineered Molecules at a Metallic Surface. Phys. Rev.Lett. 2007, 99, 038301.(44) Zhou, X. L.; Zhu, X. Y.; White, J. M. Photochemistry atAdsorbate/Metal Interfaces. Surf. Sci. Rep. 1991, 13, 73−220.



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