Nature Materials 8, 68 - 75 (2009)
Published online: 7 December 2008 | doi:10.1038/nmat2336

Subject Categories: Composites | Optical, photonic and optoelectronic materials

A synergistic assembly of nanoscale lamellar photoconductor hybrids

Marina Sofos1,5, Joshua Goldberger2,5, David A. Stone2, Jonathan E. Allen1, Qing Ma3, David J. Herman1, Wei-Wen Tsai2, Lincoln J. Lauhon1 & Samuel I. Stupp1,2,4

Highly ordered nanostructured organic/inorganic hybrids offer chemical tunability, novel functionalities and enhanced performance over their individual components. Hybrids of complementary p-type organic and n-type inorganic components have attracted interest in optoelectronics, where high-efficiency devices with minimal cost are desired. We demonstrate here self-assembly of a lamellar hybrid containing periodic and alternating 1-nm-thick sheets of polycrystalline ZnO separated by 2–3 nm layers of conjugated molecules, directly onto an electrode. Initially the electrodeposited inorganic is Zn(OH)2, but pipi interactions among conjugated molecules stabilize synergistically the periodic nanostructure as it converts to ZnO at 150 °C. As photoconductors, normalized detectivities (D*) greater than 2times1010 Jones, photocurrent gains of 120 at 1.2 V mum-1 and dynamic ranges greater than 60 dB are observed on selective excitation of the organic. These are among the highest values measured for organic, hybrid and amorphous silicon, making them technologically competitive as low-power, wavelength-tunable, flexible and environmentally benign photoconductors.

The ever-growing use of photodetectors, in a wide range of both everyday and high-end applications, has flourished in the digital age. Excellent photodetection performance is realized with sensors integrated into complementary metal–oxide–semiconductor devices, and fabricated from semiconductors such as expensive, highly engineered crystalline silicon or biologically hazardous Cd-, Pb-, Hg- and As-based compounds. The development of efficient, environmentally benign, wavelength-tunable photoconductor materials can provide new applications in flexible, low-cost, lightweight and disposable photodetectors. Organics are naturally suited to meet these requirements. When used alone, however, organics are often susceptible to device degradation through charge trapping and photo-oxidation.

Achieving highly ordered nanostructured organic/inorganic hybrids in which both components contribute to overall functionality is critical for optimizing optoelectronic device performance1, 2. For example, to make highly sensitive polycrystalline photoconductors, it is necessary to maximize the density of long-lifetime trap states that enable photoconductive gain while minimizing transport noise associated with trap states at the interfaces between crystallites3, 4. Recent studies have shown that ultrasensitive photodetectors can be made from fused PbS nanoparticles, provided that their trap states do not exist at nanoparticle grain boundaries and disrupt their conductivity4, 5. Incorporating organic dyes into the network can offer extra benefits such as tuning the optical photoaction spectra and the lifetime of trap states with molecular structure. On photoexcitation, electrons are injected from the organic into the inorganic, with the remaining hole on the organic serving as a long-lifetime trap state. Optimum sensitivity is expected by controlling the assembly and nanostructure of the two components to maximize the density of organic dye bound to the surface of nanoparticles and to minimally disrupt conduction through the percolating network.

Many successful strategies have been developed that incorporate the interactions of a structure-directing organic to template ordered nanoscale morphologies of a conductive inorganic phase6, 7, 8, 9, 10, 11. In these examples, the organic is not electronically active and remains solely to maintain the overall nanostructure. An extension of these strategies is to remove and replace the structure-directing organic with functional organic components after templating12, 13. This provides little control over the order parameter in the functional organic phase because it does not participate in the mineralization process.

Self-assembly of organic systems is desirable for enhancing hole conduction or optical properties owing to improved pipi stacking14, 15, 16, 17. In this context, we have recently explored templating insulating silica frameworks using surfactants containing electronically active conjugated moieties18, 19. The conjugated segments controlled the assembly and enhanced long-range ordering of hybrids owing to their tendency to pi-stack. To accomplish our goal of synthesizing hybrid nanostructures having both electronically active organic and inorganic components, we were inspired by recent studies of electrodeposited binary semiconductors templated by alkylated surfactants. Specifically, the synthesis of lamellar morphologies of ZnO have been claimed, but there is a dispute over the identity of the inorganic phase formed (the semiconductor ZnO versus the insulator Zn(OH)2 (refs 20–23)). ZnO is a well-established material for both photoconduction and photovoltaics,24, 25 and thiophene-based polymers and oligomers are also attractive candidates for optoelectronics. It is also known that efficient hole conduction can occur in single monolayers of thiophene oligomers26. From their relative conduction/valence bands and highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels, ZnO and most thiophene derivatives typically form type-II interfaces with rapid and efficient charge separation27. For these reasons, ZnO/thiophene hybrids have shown promise in photovoltaics and photoconduction28, 29. Electrodeposition is an attractive technique to synthesize hybrids because the temperature range is compatible with organics. Here, we couple this technique with the principles of self-assembly to synthesize lamellar hybrids with strongly interacting amphiphilic molecules containing optoelectronic functionality. These films are grown on the surface of an electrode, enabling their direct integration into functional photoconductor devices.

We grew our hybrid nanostructures on working electrodes through cathodic deposition from a H2O/dimethylsulphoxide (DMSO) solution of Zn(NO)3dot6H2O and surfactant. The surfactants used are shown in Table 1. All contain a functional group that can bind to zinc ions in the inorganic phase, and their hydrophobic segment is either an alkyl group or a conjugated moiety (pyrene or oligothiophene).

Figure 1a shows a scanning electron microscopy (SEM) image of as-deposited platelet structures using 1-pyrenebutyric acid (PyBA) as the surfactant. For a typical synthesis, these sheets are homogeneously deposited across the entire substrate, having 10–50 nm thicknesses and 1–5 mum lengths and widths. In cross-sectional SEM (Fig. 1b), the nanostructures are randomly oriented growing off the surface with the largest structures oriented in a near-vertical direction. Transmission electron microscopy (TEM) (Fig. 1c) reveals that these sheets have a lamellar morphology, where the normal of each lamella is the same as that of the macroscopic platelet. The structure has a periodicity of 3.2 nm and comprises alternating inorganic layers (dark), 0.7–1.0 nm in width, and organic layers (bright), 2.4 nm in width, corresponding to a bilayer of PyBA molecules (Fig. 1d). Small-angle X-ray scattering (SAXS) confirmed the highly ordered lamellar structure with d-spacings of 3.2 nm, 1.6 nm and 1.1 nm corresponding to (001), (002) and (003) reflections, respectively (Fig. 2a). Fourier-transform infrared spectroscopy (FTIR) (see Supplementary Information, Fig. S1) showed the presence of PyBA carboxylate stretching frequencies (1,550 cm-1 and 1,402 cm-1) and a broad O–H stretching band (3,200–3,500 cm-1), implying the surfactant is bound as a carboxylate to an–OH-containing lattice.

Figure 1: Morphology of lamellar hybrid nanostructure.

Figure 1 : Morphology of lamellar hybrid nanostructure.

a, SEM (top-down) image of deposited platelets using PyBA, magnified in the inset. b, SEM (cross-sectional view) image of deposited platelets using PyBA. c, TEM image of lamellar sheets within platelets deposited using PyBA, magnified in the inset. d, Schematic diagram of lamellar ordering composed of inorganic Zn-rich regions and bilayers of PyBA.

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Figure 2: Identity of inorganic phase and effect of annealing.

Figure 2 : Identity of inorganic phase and effect of annealing.

a, XRD patterns of hybrid films deposited with PyBA as a function of heat treatment (pre-anneal, 150 °C and 350 °C). The d001 peak increased in width, corresponding to a decrease in the domain size, d, from 90 to 20 nm based on the Debye–Scherrer equation (d=0.9lambda/Fcostheta, F: full-width at half-maximum). b, XANES spectra of hybrid films deposited with PyBA as a function of heat treatment (pre-anneal, 150 °C and 350 °C) and of Zn(OH)2 and ZnO standards. c, Local structure of the 150 °C-annealed sample (fast Fourier transform of the extended X-ray absorption fine structure), compared with that of nano-ZnO. d, High-resolution TEM image and electron diffraction pattern of hybrid film deposited with PyBA before heat treatment (Zn(OH)2). The broad 2.5–2.8 Å lattice spacing oriented normal to the plane, and the sharper 1.7 Å and 2.9 Å spacings parallel to the plane indicate diffraction from a confined two-dimensional lattice. These peak spacings do not correspond with any known Zn(OH)2 polymorph, although the crystal structures of the layered beta and delta polymorphs have yet to be to solved. e, TEM image and electron diffraction pattern of hybrid film deposited with PyBA on 150 °C heat treatment (ZnO). f, TEM image and electron diffraction pattern of hybrid film deposited with PyBA on 350 °C heat treatment (ZnO). The electron diffraction patterns after annealing (e,f) are indexed to ZnO.

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To identify the stoichiometry of the inorganic phase, temperature-dependent studies were carried out on as-deposited films using PyBA. Thermogravimetric analysis (see Supplementary Information, Fig. S2) revealed two distinct mass-loss transitions, a 20 wt% loss above 100 °C and a decomposition onset of the surfactant above 300 °C. We therefore characterized films annealed at 150 °C, and found that the carboxylate anion stretching bands in the FTIR (see Supplementary Information, Fig. S1) are still present whereas the O–H stretching band is absent. After annealing to 350 °C (see Supplementary Information, Methods, Sample characterization), the FTIR resembles the spectrum of ZnO nanoparticles. The thermogravimetric analysis thus reveals decomposition of the organic, and the FTIR suggests a transformation occurs within the inorganic phase below 150 °C.

To determine the identity of the inorganic phase at specific temperatures, X-ray absorption spectroscopy (XAS) measurements were collected. The X-ray Zn–K absorption near-edge structure (XANES) and energy position for the as-deposited film is consistent with that of a layered alpha-Zn(OH)2 polymorph standard (Fig. 2b). On annealing the film to 150 °C, the XANES reveals the transformation of Zn(OH)2 to ZnO with a structure consistent with a ZnO nanoparticle standard. In the Fourier transform (Fig. 2c) and simulation (see Supplementary Information, Fig. S3) of the extended X-ray absorption fine structure, little difference is observed for the nearest-neighbour Zn–O bonding distance (from R) and coordination number (from |chi(R)|) between the ZnO standard and the annealed samples. Therefore, the annealed samples are composed of tetrahedral Zn–O units.

Interestingly, we found that in hybrids containing PyBA the lamellar structure is preserved during the 100–150°C conversion30 from Zn(OH)2 to ZnO. The d001 spacing is present, but the higher-order d-spacings disappear in the SAXS of the 150 °C-annealed film (Fig. 2a), indicating a decrease in crystallinity of the lamellar ordering. The d001 peak is maintained after annealing at 200 °C and 250 °C, but annealing at 300 °C leads to a collapse of the lamellar architecture due to decomposition of the surfactant. High-resolution TEM and electron diffraction of the unannealed films shows that Zn(OH)2 is polycrystalline with a preferred orientation (see Fig. 2d). On annealing to 150 °C, the TEM confirms the retention of the lamellar phase throughout the entire sample (Fig. 2e). In addition, a small buckling of the lamellar sheets is apparent, probably resulting from volume contraction due to water loss. Therefore, the TEM images may not accurately show if the lamellar ZnO pathways are continuous throughout a single sheet because TEM contrast is strongly dependent on sample orientation and crystallinity. Furthermore, the electron diffraction pattern in Fig. 2e confirms Zn(OH)2 conversion to wurtzite ZnO with no preferential orientation. The intensity of the wurtzite rings is weak, indicative of the <1 nm ZnO polycrystallite size; however, the presence of (002) (101) and (100) lattice spacings can be resolved in the high-resolution TEM (see Supplementary Information, Fig. S4). On annealing to 350 °C, the lamellar morphology is replaced by larger-grain (diameter>5 nm) ZnO nanoparticles (Fig. 2f) with a more well-defined wurtzite crystal electron diffraction pattern. This temperature-dependent behaviour is reflected in the wide-angle X-ray scattering (see Supplementary Information, Fig. S5).

In contrast to the sol–gel synthesis of mesoporous oxides31, the organic in our system is necessary to retain the lamellar morphology when Zn(OH)2 transforms to the semiconducting oxide. To elucidate the role of conjugated surfactants on nanostructure integrity, we electrodeposited films using the previously studied, non-conjugated sodium dodecyl sulphate20, 21, 32 (SDS). In these films, the XANES (see Supplementary Information, Fig. S6a) indicates that the insulator Zn(OH)2, not the semiconductor ZnO, predominates at room temperature. However, annealing to 150 °C leads to a complete collapse of the lamellar nanostructure (see Supplementary Information, Fig. S6b), consistent with previous reports20. To determine if the head group influences the nanostructure stability, we electrodeposited with decanoic acid. In this case, the lamellar morphology also decomposes on annealing to 150 °C (see Supplementary Information, Fig. S7a). Therefore, pipi stacking interactions among the surfactant molecules and not binding strength between the surfactant head group and the inorganic framework is crucial to the thermal stability of the lamellar architecture during conversion. Indeed, hybrid films containing different conjugated surfactants, including pyrene carboxylic acid and terthiophene carboxylic acid (Table 1), also retain the lamellar nanostructure after annealing (see Supplementary Information, Figs S7,S8). In previous electrodeposition studies of ZnO using conjugated surfactants, the same molecular-scale lamellar structures were not achieved. This may be due to the use of a significantly lower (two orders of magnitude) molar ratio of surfactant/Zn, or the chemical structures of the surfactants33.

Extremely dense, vertically oriented growth of these lamellar hybrids can be attained with optimal growth conditions (see Supplementary Information, Fig. S9), enabling their integration into numerous optoelectronic applications. First, we must address whether electron conduction pathways can be formed in the <1 nm ZnO layers. We carried out two-terminal measurements on bulk thin-film samples (Fig. 3a,b). IV measurements of the unannealed Zn(OH)2/PyBA film showed no significant current above noise (r>1012 Omega cm), even under optical excitation with ultraviolet light. On annealing at 150 °C, a nine order of magnitude increase in conductivity was observed, revealing ohmic behaviour. The average resistivity from 10 devices is 8.1plusminus1.9times103 Omega cm (see Supplementary Information, Methods, Device testing), similar to previous values of sintered polycrystalline ZnO (ref. 34). A significant photoconductive response is observed using above ZnO bandgap light (Fig. 3c). In addition, the fluorescence of the pyrene excimer is quenched by two orders of magnitude on thermal conversion to the oxide (Fig. 3d), in contrast with the intense fluorescence observed for crystalline pyrene35. Control fluorescence and 1H NMR experiments suggest that this photoluminescence quenching is not due to oxidation of the PyBA (see Supplementary Information, Methods, PyBA photoluminescence quenching control measurement). This quenching is therefore indicative of electron transfer from the pyrene excited state into the ZnO lattice, consistent with the energy level diagram (Fig. 3d, inset)36.

Figure 3: Optoelectronic properties of hybrid PyBA nanostructures.

Figure 3 : Optoelectronic properties of hybrid PyBA nanostructures.

a, Schematic diagram of fabricated devices. PMMA: poly(methyl) methacrylate. b, SEM (cross-sectional view) image of final device. cIV curves of deposited hybrid films with PyBA before (Zn(OH)2) and after 150 °C annealing (ZnO) under dark and ultraviolet light (lambda=365 nm). d, Photographs of devices (top) and photoluminescence spectra (bottom) on 365 nm excitation before (blue) and after 150 °C annealing (red). Inset: Energy level diagram of pyrene and ZnO system.

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These studies show that the <1 nm polycrystalline ZnO matrix forms excellent conduction pathways for charge transport. The spectral responsivity of ZnO/PyBA hybrids (see Supplementary Information, Fig. S10) shows a peak response below 370 nm with no detectable photoresponse above 420 nm. This lack of visible photoresponse is expected because pyrene absorption occurs at energies higher than the ZnO bandgap (3.37 eV, 367 nm). To extend the usefulness of this lamellar architecture and prove that the organic has a functional role for photodetection, we explored tuning the spectral responsivity by synthesizing novel surfactants. Therefore, a surfactant containing a conjugated moiety with an absorption energy onset below the ZnO bandgap was designed. We synthesized a dicarboxylic acid 3-methyl-quinquethiophene (5TmDCA) (see Supplementary Information, Fig. S11), designed as a bola-amphiphile to aid in solubility. Interestingly, electrodeposition using this surfactant resulted in fibre-like (Fig. 4a) rather than previously observed flake-like morphologies. Scanning TEM (Fig. 4a) and SAXS (see Supplementary Information, Fig. S7) confirmed well-defined lamellar ordering with a periodicity of 2.5 nm that is retained on annealing above 150 °C. The smaller spacing is attributed to the organic region being composed of single layers, because each molecule has two carboxylic acid binding sites. On the basis of the 2.4 nm extended molecular length, we suggest that the molecules pack in a tilted herring-bone structure, as is well documented for thiophene oligomers in lamellar packing motifs37. The presence of sulphur in the organic enabled us to determine the surfactant/Zn stoichiometry through energy-dispersive X-ray spectroscopy (EDS) analysis. EDS on 10 lamellar sheets yielded a S/Zn atomic ratio corresponding to 1.04plusminus0.05 organic molecules per atom of Zn. There is no statistically significant change in the observed 5TmDCA/Zn (1.013plusminus0.019) ratio after annealing at 150 °C. To the best of our knowledge, this corresponds to the largest reported loading density of chromophores bound to a polycrystalline nanoparticle network, highlighting the large number of surface atoms in this system. Achieving this sensitization ratio with a network of unfused spherical ZnO nanoparticles would require using 1.4-nm-diameter nanoparticles, the synthesis of which has not been reported.

Figure 4: Structure and photoconducting properties of resulting nanostructures using 5TmDCA.

Figure 4 : Structure and photoconducting properties of resulting nanostructures using 5TmDCA.

a, SEM image of deposited fibres. Inset: Scanning TEM image of lamellar ordering composed of alternating inorganic Zn-rich (light) and quinquethiophene (dark) layers. b, IV curves of a photoconducting device measured in dark and under white light (approx100 mW cm-2). Top inset: Photograph of a fabricated device. Bottom inset: Time-dependent conductivity measured at 1 V showing photoresponse on excitation with 500 nm light (410 muW cm-2). c, Ultraviolet–visible absorption and spectral responsivity (left and right vertical axis respectively as indicated by arrows) measured at 1 V. For the spectral responsivity, the irradiance at each wavelength ranged between 160 and 440 muW cm-2 depending on the wavelength. Inset: Energy level diagram of the 5TmDCA and ZnO system showing electron transfer. d, Photocurrent versus optical illumination, revealing a dynamic range of greater than three orders of magnitude or 60 dB, as measured at 500 nm excitation. e, Gain and detectivity (left and right vertical axis respectively as indicated by arrows) as a function of wavelength, measured at 1 V. The irradiance at each wavelength ranged between 8 and 22 muW cm-2 for all wavelengths. Inset: Gain versus irradiance for the 500 nm wavelength under these same conditions. f, Detectivity measured at 2 V and 500 nm excitation (4 muW cm-2) as a function of modulation frequency.

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Two-terminal ZnO/5TmDCA photoconductor devices were fabricated similar to the ZnO/PyBA hybrids (Fig. 4b, top inset). The average dark resistivity from seven devices is 4.8plusminus2.2times104 Omega cm. A pronounced photocurrent response is observed on excitation with white light (Fig. 4b). However, a significant increase in the spectral responsivity is observed between approx400 and 580 nm, coincident with the absorption of the quinquethiophene (Fig. 4c and Supplementary Information, Methods, Device testing). This visible photoresponse occurs with both rapid (<1 s) and slow (>300 s) rise and decay times (Fig. 4b, bottom inset). This slow time response is commonly observed in ZnO photoconductivity and originates from the release and recapture of chemisorbed oxygen molecules on reacting with a hole (O2-(ads)+h+right arrowO2(g)). Release of chemisorbed oxygen heavily dopes the ZnO surface, which prevents charge-carrier recombination and prolongs the photocarrier lifetime. In our system, selective photoexcitation of the quinquethiophene leads to an increase in photocurrent through multiple processes. Initially, electron transfer occurs rapidly from the dye into the ZnO, consistent with the energy level diagram (Fig. 4c, inset). Then, the remaining hole on the quinquethiophene relaxes through capturing an electron from a surface chemisorbed oxygen, resulting in the slower photoconductive response38.

The 500 nm photocurrent response varies over three orders of magnitude of incident intensity, corresponding to a dynamic range greater than 60 dB (as defined in imaging applications; Fig. 4d) and comparable to current complementary metal–oxide–semiconductor image sensors. Photoconductive gain is defined as the ratio of the number of photocurrent electrons per time to the number of incident photons per time and derived from the spectral responsivity (Fig. 4e). A high gain is required for achieving sensitive detection of an optical signal, which is desirable for low-light measurements. When selectively exciting the organic (lambda=500 nm), the photoconductive gain is above unity (100%) and typically ranges from 1.2 to 120 at low electric fields (1.2 V mum-1), depending on light intensity (Fig. 4e, inset). The decrease in gain at higher intensities is due to saturation of the photoexcited trap states39.

The photodetector sensitivity (or detectivity) refers to the minimum optical signal that can be distinguished above noise. The dark-current density for a typical 5TmDCA/ZnO photoconductor at 2.4 V mum-1 is approx17 mA cm-2. From these values, the Johnson noise is estimated from (4kTB/R)1/2 to be approx10 pA Hz-1/2 and the shot noise (2qId/B)1/2 is approx61 pA Hz-1/2, where k is Boltzmann's constant, T is temperature, B is the noise bandwidth, R is the dark resistance of the detector, q is the electron charge and Id is the dark current. The 2 V bandwidth-normalized noise current In*of these photoconductors was directly measured to be approx130 pA Hz-1/2, which is about two times the shot noise limit. The area-normalized detectivity D* in Jones (cm2 Hz1/2 W-1) is calculated from D*=A1/2SR/In*, where A is the device area and SR is the spectral responsivity (Fig. 4e). Figure 4f shows the detectivity response under modulated illumination. At 500 nm excitation, 1 Hz and 2 V bias, D* is observed to be 7.0times109 Jones. At slower modulation frequencies (0.25 Hz), the 500 nm D* is found to increase to approx2.0times1010 Jones with little apparent dropoff. Although we focused on the performance due to the organic, the D* values increase by a factor of about five when exciting ZnO (370 nm).

To the best of our knowledge, these room-temperature, low-bias (<2 V), 500 nm excitation gain and detectivity values are the highest measured for organic or hybrid organic/inorganic photoconductors40, 41, 42. Furthermore, the 30 Hz D* (approx8.2times108 Jones) is approximately equal to the best reported amorphous silicon photodetectors43. Although these values are not as high as the recently reported inorganic PbS nanoparticle photodetectors4, 5, these initial measurements are quite promising and we expect further enhancements with appropriate device engineering, for example by reducing the noise and dark current through changing the device dimensions or contact metal choice. Our devices remained stable after repeated measurement and storage in air for eight months, with virtually no change in dark current or responsivity.

To determine the influence of the lamellar nanostructure on photoconduction, we prepared films of 10 nm ZnO nanoparticles, sintered to guarantee ZnO conduction, and functionalized with 5TmDCA (see Supplementary Information, Fig. S12). From EDS, about 2.5plusminus0.3 5TmDCA molecules are bound per 100 Zn atoms, corresponding to approx31% surface coverage of the nanoparticles. Although the 5TmDCA:Zn ratio is much lower, approx80% of the 500 nm light is absorbed in one pass, according to the ultraviolet–visible absorption spectra (see Supplementary Information, Fig. S12c). At the ZnO bandgap, the spectral responsivity of the lamellar device is only approx10% greater than the control (see Supplementary Information, Fig. S12d). However, from 400 to 560 nm, the lamellar devices exhibit spectral responsivities 400–600% greater than the control. This increase cannot be explained by absorption alone; the lamellar architecture must either enhance majority-carrier and/or trap-state lifetimes or reduce majority-carrier transit time. A detailed comparison of the trap-state and carrier lifetimes and mobilities will help elucidate the actual enhancement mechanism.

We conclude that hybrid electronically active materials consisting of nanoscale alternating layers of inorganic and organic components are attained in essentially one step by integrating molecular self-assembly and electrodeposition. The presence of conjugated moieties in the organic not only adds an optoelectronic component to the hybrid, but also synergistically stabilizes the lamellar architecture on thermal conversion of the insulator Zn(OH)2 to the semiconductor ZnO. This biologically non-toxic hybrid combines the spectral tunability of an organic with the superior conductivity and protective encapsulation of an inorganic to attain stable photoconductive performance. This method offers a promising approach towards the synthesis and device integration of previously inaccessible bifunctional p-type/n-type hybrid materials with lamellar architecture. The strength of the pipi interactions in preserving this lamellar nanostructure suggests the presence of conducting pathways in both the organic and inorganic layers, making this type of structure ideal for other applications including photovoltaics.



Organic surfactant synthesis.

The terthiophene carboxylic acid was synthesized according to the literature, replacing the base used, lithium di-isopropylamide, with n-butyl lithium44. Synthetic details of 5TmDCA are in Supplementary Information. The remaining surfactants, PyBA (Aldrich, 97%), decanoic acid (Aldrich, 99%), 1-pyrenecarboxylic acid (Alfa Aesar, 98%) and SDS (J.T. Baker, ultrapure bioreagent), were all available commercially. Extended molecular lengths were calculated using Molecular Mechanics minimization with the MM+ force field in Hyperchem.

Electrochemical synthesis.

Electrochemical syntheses were carried out in 6 ml solutions (1:1 (v/v) DMSO/H2O) of 0.02 M Zn(NO)3dot6H2O (Aldrich, 98%) with 1.5–3.0 mg surfactant, purged for 20 min with Ar, in an undivided cell vial using a three-electrode set-up (EG&G Princeton Applied Research potentiostat model 263A) with a Zn counter electrode (Alfa Aesar, 99.9997%) and a Ag/AgCl reference electrode (BAS Inc. model RE-5B). Water is required for reduction of NO3- to generate OH-, and DMSO is added for surfactant solubility. Working electrode substrates (indium tin oxide (ITO), fluorine tin oxide or Ag) were placed upright in the cell and deposition was achieved potentiostatically at -0.9 V for 30 min–24 h, depending on growth time, under constant magnetic stirring at 80 °C. Deposited films were rinsed with water and ethanol and dried with dry N2. For device measurements, the working electrodes were ultraviolet–ozone-treated 2.0 cm2 glass substrates with pre-patterned ITO (140 nm thick, approx50 Omega sheet resistance, Kintek). For all other characterization, 2.5 cmtimes2 cm ITO–glass (120 nm thick, approx100 Omega sheet resistance, Applied Thin Films) or fluorine tin oxide/glass (TEC#8/3, approxOmega sheet resistance, Hartford Glass) electrodes were used.

Synthesis of powder standards.

ZnO nanoparticles (Nanotek, 99.999%) were used as a standard for FTIR, powder XRD and XAS measurements. Nanoparticles for control measurements were synthesized as reported by Pacholski et al.45. The alpha-Zn(OH)2 standard was synthesized as reported by Ogata et al.23. Briefly, a 60 ml 0.17 M aqueous solution of Zn(NO)3dot6H2O (Aldrich, 99.9%) was stirred at 5 °C and a 36 ml 0.23 M NaOH solution was prepared. The solutions were stirred together at 5 °C for 1 h and a cloudy white suspension was obtained. The product was filtered, washed three times with H2O and air-dried at room temperature for 24 h. X-ray diffraction (XRD) analysis showed a homogeneous single phase of alpha-Zn(OH)2 nitrate (actual stoichiometry: Zn5(OH)8(NO3)2dot2H2O) (ref. 30).

Device fabrication.

For conductivity measurements, a 500-nm–2-mum-thick active layer of ZnO templated with either PyBA or 5TmDCA was electrochemically deposited onto a pre-patterned ITO anode (15 ohm/square). The ZnO/5TmDCA was annealed under vacuum at 150 °C for 12 h. A 2-mum-thick photoresist protective layer (poly(methyl) methacrylate, Microchem Corp.) was spin-cast and either dried under vacuum for 4–24 h for the PyBA, or annealed for 10 min on a 150 °C hot plate for the 5TmDCA, to prevent shorting. Substrates were reactive-ion-etch cleaned in 50 s.c.c.m. O2 at 100 W until the tips of the ZnO sheets were observed by SEM. The samples were then reactive-ion-etch cleaned in 50 s.c.c.m. O2 at 50 W for 1 min and loaded into an electron beam evaporator equipped with an in situ temperature sensor for evaporation of a top electrode, 400 nm/150 nm (Ti/Ag or Au), through a shadow mask, resulting in a 2 mmtimes2 mm active area for each device. The maximum temperature during evaporation was 61 °C.

For the control nanoparticle devices, a mixture consisting of 6 wt% ZnO nanoparticles and 3% polyethylene glycol (Mw=1,000) in water was ultrasonicated for 2 h to create a slurry. The slurry was doctor-bladed onto pre-patterned ITO electrodes masked with a single layer of scotch tape, and sintered at 400 °C for 30 min. Sensitization occurred by soaking in a 1 mM solution of 5TmDCA in DMSO for three days. To remove residual molecules, substrates were soaked in DMSO for 15 min and rinsed with methanol three times. A 400 nm/150 nm Ti/Ag top electrode was evaporated on top using a shadow mask with a 2 mmtimes2 mm active area for each device.

The initial thicknesses of deposited films were measured using a Tencor P10 profilometer. After fabrication and testing, device active area thicknesses were measured by cross-sectional SEM, and ranged from 700 nm–1.2 mum for the PyBA samples, 700–800 nm for the 5TmDCA samples and 800 nm–1 mum for the control device.

Details of the sample characterization, surface atom estimate for ZnO nanoparticles, optical measurements, PyBA photoluminescence quenching control measurement, 5TmDCA energy level determination and device testing are in Supplementary Information, Methods.



This work was supported by the US Department of Energy under Award DE-FG02-00ER54810 and the National Science Foundation under award DMR 0605427. Experiments made use of the following facilities at Northwestern University: J. B. Cohen X-ray Diffraction Facility, IMSERC, the EPIC and Keck-II Facilities of the NUANCE Center, the Keck Biophysics Facility and the Institute for BioNanotechnology in Medicine. The NUANCE Center is supported by the NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois and Northwestern University. We acknowledge facilities support by the Materials Research Center through NSF-MRSEC grant DMR-0520513. XAS measurements were carried out at the DuPont–Northwestern–Dow Collaborative Access Team (DND-CAT) Synchrotron Research Center located at Sector 5 of the Advanced Photon Source. DND-CAT is supported by the E.I. DuPont de Nemours & Co., The Dow Chemical Company, the US National Science Foundation through Grant DMR-9304725 and the State of Illinois through the Department of Commerce and the Board of Higher Education Grant IBHE HECA NWU 96. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-Eng-38. We also thank L. Palmer for useful discussions.

Received 19 March 2008; Accepted 28 October 2008; Published online 7 December 2008.



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  1. Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA
  2. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA
  3. DND-CAT, Northwestern University Synchrotron Research Center, 9700 South Cass Avenue, Argonne, Illinois 60439, USA
  4. Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, USA
  5. These authors contributed equally to this work

Correspondence to: Samuel I. Stupp1,2,4 e-mail:


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Nature Materials News and Views (01 Jan 2009)