Metal Octaethylporphyrin Nanowire Array and Network toward Electric/PhotoelectricDevices

Jin-Song Hu, Heng-Xing Ji, and Li-Jun Wan*Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences(CAS), Beijing 100190, China

ReceiVed: June 21, 2009

A vapor transfer deposition method was developed to fabricate metal (metal ) Co, Ni, Cu, Zn, Mg)octaethylporphyrin (MOEP) nanowire arrays in large area on a variety of substrates. The formation processof MOEP nanowire arrays was investigated by time-dependently terminating deposition at various stages,based on which, a vaporization-condensation-recrystallization (VCR) mechanism was proposed to understandthe formation of nanowires and thus guide the synthesis of three-dimensional (3D) sea urchin-like nanowireassemblies and two-dimensional (2D) nanowire networks. As application examples of these porphyrinnanostructures, the porphyrin nanowires demonstrated a good field emission property and the photoelectricdevice based on the 2D MOEP nanowire network was fabricated, showing a good light-induced signalamplification behavior.

Introduction

The controllable fabrication of nanoscale objects with well-defined morphologies and sizes is essential for the understandingand development of novel potentially useful materials, and thebasic requirement for fabricating nanodevices aimed at variousapplications. Extensive efforts have been devoted to developingsynthetic strategy to prepare inorganic nanomaterials withtunable morphologies and structures, and thus with tailoredelectronic and optical properties.1-5 Assembling organic func-tional molecules including biomolecules, organic small mol-ecules, and polymer or oligomer into nanostructures, especiallyone-dimensional (1D) ones, has recently attracted much attentionand is of special interest due to common building principlesfound in both living and nonliving systems, the tunableelectronic and optical properties, as well as greater variety andflexibility of these materials via molecular design.6-12 Theseorganic nanostructures have shown surprising electronic, optical,and photoelectric properties as well as great potentials in diverseapplication fields such as molecular electronics, photonics, light-energy conversion, and catalysis.13-19 So far, however, the abilityto understand and control the growth of 1D organic nanostruc-tures and their higher order assemblies still needs to be furtherimproved in contrast to their inorganic counterparts.20-23

As important macrocyclic complexes, metalloporphyrins havebeen widely studied in the fields ranging from chemistry, physics,biology, and medicine to molecular device due to their easysynthesis and abundant and diverse properties, especially thetunability of properties via substitution by various functionalgroups.24 One of the very active research fields is to fabricatedesignable porphyrin molecular wires or supramolecular self-assemblies by covalent or noncovalent linkage with the motivationof studying the importance of interchromophore distance andorientation in intra- or intermolecular energy transfer and/or electrontransfer reactions, as well as being effective biomimetic modelsfor light harvesting.25-28 Recently, some metalloporphyrin nano-structures have been prepared by the supramolecular self-assembly

process induced by solvent evaporation, solubility changes, ionicinteraction, etc.,29-33 which have shown the attractive applicationpotentials in photoelectrical conversion, photocatalytic water-splitting, and photoelectronic nanodevices.26,33-38 There is no doubtthat the development of these metalloporphyrin nanostructures willexpend the research fields of porphyrin compounds and make animportant step toward the construction of functional nanoscaledevices and systems based on them. In this paper, we developedan improved vapor transfer deposition method for allowing us tofacilely control and investigate the growth process of nanomaterials.As a result, metal (metal ) Co, Ni, Cu, Zn, Mg) octaethylporphyrin(MOEP) nanowire arrays were synthesized and their formationprocess could be successfully investigated due to the ability toterminate growth during the deposition at the desirable time, basedon which a vaporization-condensation-recrystallization (VCR)mechanism for organic nanowire growth was proposed and 2DMOEP nanowire networks and 3D sea urchin-like nanowireassemblies were fabricated under the inspiration of this mechanism.Furthermore, for demonstrating the potential applications of theseporphyrin nanostructures, the stable and good field emissionproperty was for the first time reported for a porphyrin nanowirearray and the photoelectric device based on 2D MOEP nanowirenetworks showing a good light-induced signal amplificationbehavior was successfully fabricated, both of which might extendthe technological application for this type of abundant, easilysynthesized, and design-on-demand porphyrin compounds.

Experimental Section

Metal (metal ) Co, Ni, Cu, Zn, Mg) octaethylporphyrin(MOEP, molecular structures were presented in Scheme 1) werepurchased from Sigma-Aldrich Chemical Co. and used directlywithout purification. The porphyrin nanostructures were fabri-cated through a vapor transfer deposition process in a one-zonehorizontal tube furnace as illustrated in Scheme 2. A quartzboat loaded with metal octaethylporphyrin powder was put intoa quartz tube that was inserted into a horizontal tube furnaceand placed at the high-temperature zone. Cooling water flowedinside the cover caps to achieve a reasonable temperaturegradient in the tube and a mechanical pump was connected to

* To whom correspondence should be addressed. E-mail:wanlijun@iccas.ac.cn.

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10.1021/jp905818n CCC: $40.75 2009 American Chemical SocietyPublished on Web 08/17/2009

evacuate the system. An iron loop was attached onto thedownstream end of the inner quartz tube, which allowed us toextract or insert substrates together with the inner tube by amagnet outside the outer quartz tube so as to terminate orcontinue the growth of organic nanostructures at any desirabletime. This design made it possible to investigate the growthstatus at any stage and to control the morphologies of theresulting nanostructures. For the synthesis of MOEP nanostruc-tures, the powder was heated to 380 °C at a rate of 10 degmin-1. Highly pure nitrogen was used as the carrier gas to carryporphyrin vapor to the low-temperature zone. The flow rate ofnitrogen was kept at 200 sccm (standard cubic centimeters perminute) and the pressure in the tube was maintained at 200 Pa.After deposition and cooling, products could be collected onvarious substrates located in the low-temperature zone wherethe temperature was 270 ( 5 °C.

Field emission scanning electron microscopy (FESEM, JEOL-6701F), Fourier transform infrared (Bruker Tensor 27 FT-IRspectrometer, KBr pellet), and X-ray diffraction (Rigaku D/max-2500, using filtered Cu KR radiation) were used to characterizethe morphologies, chemical compositions, and crystal structuresof MOEP samples, respectively.

For fabricating phototransistors, a 2D nanowire network wasdeposited on Si/SiO2 (500 nm) followed by thermal evaporationof a 50 nm gold contact with a shadow mask. The two-probemethod was applied to probe the photoeffect on the electricalproperties of CoOEP nanowire network. The I-V curve wasmeasured with a Keithley 4200 SCS and Micromanipulator 4060probe station at room temperature in air. A white-lightiodine-tungsten lamp with a power intensity ranging from 0.7to 8.9 mW · cm-2 was applied as the illumination source.

The field emission properties were measured in a vacuumchamber of 5 × 10-7 Pa at room temperature under a two-parallel-plate configuration. The MOEP nanowire array directlygrown onto a silver-coated silicon slice with an area of 0.4 cm2

was mounted onto a stainless-steel sample stage with use ofconducting glue as the cathode. The anode was another parallelstainless-steel plate. The distance between the two electrodeswas 150 µm.

Results and Discussion

Cobalt octaethylphorphyrin (CoOEP) was hereinafter selectedas a typical sample to illustrate the morphologies, structure, andgrowth process of metal octaethylphorphyrin nanostructuressynthesized by an improved vapor transfer deposition methodas mentioned in the Experimental Section. After 20 min ofdeposition, the nanowire array can be obtained as shown inFigure 1a. The sample comprises a number of nanowiresstanding on the substrate ca. 200 nm in width and ca. 5 µm inlength. The average length of the nanowires can be readily tunedby changing deposition time. For example, as shown in Figure2, they are ca. 2 µm long in the case of 10 min of depositionwhile they can reach ca. 10 µm in length after 40 min ofdeposition. As seen from the enlarged SEM image in Figure1b, most of these nanowires have regular shapes with smoothsurfaces and rectangular cross sections. The area of the nanowirearray is dependent on the size of the substrate (Figure 1c), whichenables wafer scale production for integration with silicon basedmicro- and nanotechnology. It should be noted that CoOEPnanowires can be prepared on a variety of substrates with similarprocess parameters in the present study, such as Si substrate,silicon dioxide or metal-coated substrate, glass, and quartz. Thenanowires grown on these substrates showed similar morphol-ogies and the same structure.

FTIR spectra were measured to reveal the chemical composi-tion of these nanowires and possible structure change after vapordeposition. By comparing FTIR spectra in Figure 3a recordedfrom CoOEP nanowires and source powder, it can be foundthe FTIR spectrum of nanowires has the same features as thatof CoOEP starting material. The four characteristic metal-sensitive IR bands of octaethylporphyrin at 750, 922, 991, and1228 cm-1 are very similar in both cases, which indicates thatCoOEP molecules did not undergo decomposition/chemicalreactions during vaporization and deposition.39 Moreover, X-raydiffraction (XRD) experiments were done to investigate thecrystalline structure of CoOEP nanowires and their preferentialgrowth orientation. The measured XRD pattern of CoOEPnanowires and the simulated one from published single crystal-line data are presented in Figure 3b.40 The diffraction peaksappearing in the XRD pattern of CoOEP nanowires show thatthe as-prepared CoOEP nanowires are highly crystallized. It isalso obvious that these peaks can be indexed well according tothe reported structure (CCDC, Refcode: HECZIY),40 and thediffraction from (kkj0) planes is significantly enhanced relativeto that in the simulated pattern, indicating that the preferentialgrowth orientation of the nanowires is perpendicular to the (kkj0)plane.40

Understanding the formation process of nanostructures is ofgreat significance in unveiling their growth mechanism and thusguiding the synthesis in a controllable way toward the desirableapplication. In the present study, we designed a retractableloading setup in our physical vapor deposition system as shownin a scheme of the system (Scheme 2), in which an iron loopwas attached onto the end of the inner quartz tube and a magnetwas used to operate the inner tube from the outside. Thisimprovement allows us to rapidly extract substrates from theheating segment and thus quench/terminate on demand thedeposition/growth of MOEP nanostructures at any desirablestages so as to make the direct examination of intermediatespossible. The SEM images of intermediate products obtainedat different stages during the deposition of CoOEP are presentedin Figure 4. After 1 min of deposition, as shown in Figure 4a,many spherical particles (100-200 nm) are formed on thesubstrate and some shaped pieces (up to 1 µm) with no/less

SCHEME 1: Molecular Structure of the Used MetalOctaethylporphyrins (MOEPs)

SCHEME 2: Schematic Illustration of the PVD Methodfor Preparing and Analyzing the Growth Process of theMOEPs Nanowires

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spherical particles around them are also found in some regions.Three minutes later (Figure 4b), the spherical particles disappearand instead interconnected strips grow from certain positionsalong the substrate surface and form 2D networks. As depositionproceeds (Figure 4c), these strips tend to stack together andcover the whole surface of the substrate. After 10 min ofdeposition, the head of these strips grows out of the substrateas nanowires (Figure 4d). It can also be found that the roots ofeach nanowire still cross closely on the substrate like a highlypacked nanowire film. Finally, freestanding nanowire arrays withlengths up to 5 µm can be obtained (Figure 4e) after 20 min ofdeposition. If the deposition time was increased to 40 min, seaurchin-like 3D nanowire assemblies with a wire length of ca.10 µm would be found on the top of the substrate (Figure 4f).On the basis of the above observation and the facts that thetype of substrate has little influence on the growth of thenanowire as well as no chemical composition change takes placeduring deposition, a vaporization-condensation-recrystallization(VCR) mechanism shown in Figure 4g can be proposed toelucidate the formation of the porphyrin nanowire array in thepresent study.41,42 Initially, the porphyrin source powder is

vaporized. The vapor is transferred by carrier gas down to thelow-temperature zone and then undergoes heterogeneous nucle-ation on the substrate there to form spherical nanoparticles.Induced by local supersaturation and temperature, these nano-particles subsequently fuse and crystallize into shaped piecesand then nanostrips which can further grow into interconnected2D networks along the substrate surface to meet the minimumenergy requirement with the supply of porphyrin molecules.When the growth space on the substrate is getting more andmore limited the nanostrips tend to protrude from the substrateas a result of preferential axial growth. As the condensationand crystallization proceed, the protruded tips are becominglonger and finally grow into nanowire arrays. When the vaporconcentration continues to increase, the molecules in vapor formcan nucleate on the surface of previously formed nanowires andfurther grow in three dimensions, leading to the 3D sea urchin-like nanowire assemblies. In the experiment, it was found ifthe amount of porphyrin source material was not enough onlya nanowire array could be achieved without these nanowireassemblies regardless of the deposition time.

On the basis of the above proposed mechanism, it can beexpected that 2D CoOEP nanowire networks with tunablecoverage can be fabricated by controlling the depositionparameter. In fact, the 2D nanostructures like those shown inpanels b and c of Figure 4 could be achieved in the normaldeposition area (with temperature at 270-280 °C) by delicatelypulling the substrate out and terminating the deposition.However, it was also found if we collected products in the zonefar from the source area with a lower temperature of 210-200°C, the 2D nanowire networks can be easily obtained within amore allowable time range. This is because the growth rate ofnanowires in the vaporization-condensation-recrystallizationprocess is supposed to mainly depend on the saturation degreeof the molecule vapor. The supersaturation of vapor wasrelatively small in downstream zone so as to achieve areasonable deposition rate for controlling nanowire networkgrowth. As a result, the products can be tuned from the

Figure 1. (a) Low and (b) high SEM images of the CoOEP nanowires. (c) Photograph of the CoOEP nanowires deposited on a quartz surface.

Figure 2. SEM images of the CoOEP nanowires collected at 10 (a) and 40 min (b).

Figure 3. (a) FTIR spectra of CoOEP nanowires (blue) and sourcepowder (brown). (b) XRD pattern of CoOEP nanowires (blue) andsimulated pattern from the single crystal data (CCDC, Refcode:HECZIY).

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interconnected CoOEP nanowire web to a highly packednanowire film as shown in Figure 5. It should be noted thatthis type of nanowire network may provide some specialproperties and novel application potentials such as in sensors,optics, photoelectrics, and electronics.10,43 For example, theoptical and electrochemical characteristics of porphyrin com-pounds are very sensitive to some particular analytes such asNOx or nitro-containing molecules.24 The interconnected nano-wire net will allow rapid long-range electron transfer viaeffective intermolecular π-electron delocalization and porousstructure will allow rapid mass transfer. It therefore can beexpected that this unique 2D network may be used as an efficientsensor for detecting NOx or explosive nitroaromatic compounds(NACs, such as TNT etc.) as well as optoelectronic devices.

The follow-up investigation is still under way. As an example,this crystalline porphyrin nanowire film has been demonstratedas a good candidate for fabricating thin film phototransistors insignal amplification, as described below.

Organic thin film transistors are envisioned as a viablealternative to traditional thin-film transistors based on inorganicmaterials and have been widely applied in flexible electronicdevices. However, most of them are based on randomlydeposited organic thin films with a large number of grainborders, which would limit charge transport during working.44,45

A two-dimensional porphyrin nanowire network combines thefeature of nanowires with high orientation and continuity andthe feature of thin films with feasibility in processing. Moreimportantly, interconnected net structure will greatly enhance

Figure 4. (a-f) Morphology evolution of CoOEP nanowires at different stages. The SEM images were taken from the samples collected byterminating deposition at 1, 3, 5, 10, 20, and 40 min respectively after deposition temperature reached 380 °C. (g) Schematic illustration of theformation of CoOEP nanostructures.

Figure 5. 2D CoOEP nanowire networks with different coverage. The deposition duration was 10 (a), 15 (b), and 20 (c) min.

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charge transport through efficient intermolecular π-electrondelocalization. Furthermore, owing to their ability to convertlight energy into electron motion, porphyrins and metallopor-phyrins are widely used in light-harvesting systems. Thesefeatures would make it a good candidate for flat electronicdevices, especially light-operating electric devices. Thus, CoOEPnanowire networks are utilized to fabricate phototransistors. Asshown in Figure 6a, gold contact electrodes with a 40 µmchannel were deposited directly on the CoOEP nanowirenetwork by a shadow mask. Figure 6b shows the outputcharacteristics of the nanowire network based device underwhite-light irradiation at various light intensities. The devicedisplays a large increase in current as enhancing the lightintensity, demonstrating that the output of the transistor can becontrolled by incident light. This output characteristic is similarto those of other organic FETs controlled by gate voltage. Thelight-sensitive current output is because a large number of chargecarriers can be generated after CoOEP nanowires absorb light

with photon energy equal to or higher than their band gapenergy, which leads to an increase in current. This resultsuggests that light can act as an additional terminal to controlthe output of the CoOEP-based transistor for signal magnification.

On the other hand, it has been reported that some semicon-ductor, metal, and some organic nanowire arrays grown onconductive substrates show stable field-emission currents be-cause electrons can be easily extracted from nanowire tips byan electric field.46-49 Herein, inspired by the structure featuresof the CoOEP nanowire array, we investigated its field emissionproperty and for the first time found that a porphyrin-basednanowire array can also emit stable current under an electricfield. To enhance electrical conductivity, the CoOEP nanowirearray was grown on a silicon substrate thermally coated with a10 nm silver thin film. Figure 7a presents the measured fieldemission characteristics in terms of the typical curve of currentdensity J vs applied electrical field E. The turn-on field is 15.4V ·µm-1 where emission current reaches 10 µA · cm-2 and themaximum emission current can reach up to 2000 µA · cm-2 atan applied field of 20.5 V ·µm-1. The emission characteristicswere analyzed by using the Fowler-Nordheim model describedas50

where A and B are constants (A ) 1.54 × 10-6 A ·V · eV, B )6.83 × 109 V ·m-1 · eV-3/2), J is current density, E is electricfield, φ is the work function of the emitting sample, and � is

Figure 6. (a) Photograph of the CoOEP nanowire network betweentwo gold contact electrodes on Si/SiO2 substrate. (b) Signal amplificationof the CoOEP nanowire network by utilizing light radiation instead ofgate voltage.

Figure 7. (a) Field emission J-E curve and the corresponding FN plot (inset). (b) Emission stability of the CoOEP nanowire array.

Figure 8. SEM images of MOEPs (M ) Ni, Cu, Zn, Mg) nanowires and their corresponding EDX profiles.

lnJ

E2) ln

A�2

φ+ -Bφ

3/2

�E

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the field enhancement factor related to the emitter geometry.The field enhancement factor can be deduced from the slope ofthe FN plot of ln(J/E2) versus 1/E, as depicted in the inset inFigure 7a. Herein, the slope k ) -539.8 and the work function,φ, can be assumed as 6.0 eV according to ultraviolet photo-emission spectroscopy (UPS) analysis, which suggests that thehighest occupied π molecule orbital of most MOEPs is between6.0 and 6.8 eV.51 Accordingly, the field enhancement factor isestimated to be ca. 186. The emission stability was conductedwith an emission current density of 500 µA · cm-2 at an appliedfield of 19.5 V ·µm-1 for 30 min. The J-t curve is presented inFigure 7b. It can be seen that no obvious degradation of currentdensity was detected and the emission current deviation wasca. 4% during a period of over 30 min of testing, indicating thehighly durable and robust current emission capability of theCoOEP nanowire array. To the best of our knowledge, such astable field emission property was for the first time reportedfor porphyrin compounds, which will extend their application.

Furthermore, it should be noted that the present fabricationmethod can be applied to other metal octaethylporphyrins(MOEPs). With no great variation in processing parameters, wehave successfully utilized this method to synthesize nanowirearrays of NiOEP, CuOEP, ZnOEP, and MgOEP as shown inFigure 8. According to SEM images, nanowire arrays can alwaysbe collected at the downstream of N2 carrier gas after 20 minof deposition. The difference in MOEP nanowire lengths mightbe due to the difference of sublimation temperatures of MOEPsassuming the same evaporation temperature was used tofabricate these nanostructures. The compositions of MOEPnanowires were analyzed by an energy dispersed X-ray (EDX)spectrometer equipped in SEM. The recorded EDX spectrumof each nanowire sample unambiguously shows the peak of themetal element (Ni, Cu, Zn, and Mg, respectively), whichoriginates from the metal center of the corresponding porphyrinmolecules. Although further investigations are underway toelucidate the effect of temperature on the morphology and sizeof different MOEP nanowires as well as compare their fieldemission properties, these initial results have demonstrated theability to fabricate different MOEP nanowire arrays by thestrategy presented in this paper.

Conclusions

In summary, metal (metal ) Co, Ni, Cu, Zn, Mg)octaethylporphyrin (MOEPs) nanowire arrays, 2D intercon-nected nanowire networks, and 3D sea urchin-like nanowireassemblies were successfully fabricated by a vapor transferdeposition method. The formation process was analyzed bytime-dependently terminating the deposition with a magneti-cally controlled retractable setup, which supported avaporization-condensation-recrystallization mechanism re-sponsible for the formation of MOEP nanostuctures. Theability of light controlled signal amplification of MOEPnanostructures was demonstrated on nanowire networks andthe field-emission behavior of the CoOEPs nanowire arraywas investigated, which might allow porphyrin nanostructuresas a candidate for light-sensitive nanodevices and stable large-area cold cathode materials in flexible electronics.

Acknowledgment. The authors thank Prof. D. P. Yu atPeking University for the measurement of field emissionproperties, and the financial support from the National NaturalScience Foundation of China (Nos. 20603041 and 20733004),NationalKeyProjectonBasicResearch(GrantNos.2006CB806100and 2006CBON0100), and the Chinese Academy of Sciences.

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Metal Octaethylporphyrin Nanostructures J. Phys. Chem. C, Vol. 113, No. 36, 2009 16265