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Chemical Tuning of the Electronic Properties of Nanostructured Semiconductor Films Formed through Surfactant Templating of Zintl Cluster Scott D. Korlann,† Andrew E. Riley,† Bongjin Simon Mun,§,‡ and Sarah H. Tolbert*,† UniVersity of California Los Angeles, Los Angeles, California 90095-1569, AdVanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: July 31, 2008; ReVised Manuscript ReceiVed: February 2, 2009
Inorganic/organic coassembly provides a powerful route to the formation of periodic, nanostructured materials. In this work, the surfactant cetyltriethylammonium bromide is used as an organic structure directing agent, and the inorganic phase is formed from the condensation of metal cations with reduced main group clusters know as Zintl clusters. These anionic clusters are formed by alloying alkali metals with various main group elements. The chalcogenide-based Zintl clusters used here have an affinity for gold and other transition metals and will thus nucleate the formation of films on metal surfaces. Interface nucleated inorganic/organic coorganization results in thin films with the periodicity of a liquid crystal phase, but with a cross-linking inorganic network surrounding the surfactant domains. In this work, we investigate the extent to which the band structure of these films can be tuned by altering the elemental composition of the inorganic framework of these periodic nanocomposites. For the semiconducting films investigated here, the band gap and valence and conduction band energies of the inorganic network can be independently tuned by 1-2 eV by varying different elemental components. All trends in the data can be qualitatively understood by considering the orbital contribution to the band structure, in analogy to chalcogenide glass semiconductors. A variety of applications are anticipated for nanostructured semiconducting films for which band properties can be independently tuned across a broad range and films can be synthesized using low cost solution phase methods. Introduction Template-directed formation of periodic inorganic/organic composite materials has proven to be an advantageous method of forming complex materials. This process involves the coassembly of organic surfactants and inorganic oligomers in solution to form a periodic mesostructured composite. The material is stabilized by polymerizing the inorganic component into a robust framework. Phases with either a hexagonal honeycomb structure or a cubic nanoscale periodicity can be produced. Silica-based mesostructured composites were among the first of the class of mesostructured composite systems.1-6 Analogous methods have been used to synthesize many transition metal oxides, such as Al2O3, MnxOy, Nb2O5, SnO2, TiO2, TiO2/VO2, ZrO2, and NiO2.7-20 Although some low-valent Mn and Nb/Ti oxide mesostructures are conducting,21,22 most oxidebased mesostructured composite materials are either insulating or have very wide band gaps and, thus, are often less desirable as the active components in many traditional semiconducting applications, such as solar cells, LEDs, and FETs. Nonoxide low-band-gap semiconductor mesostructured composites exhibited promise for these applications because they can be formed with band gaps ranging from many electronvolts to less than 1 eV.23-33 These cubic or honeycomb structured inorganic/organic composites are formed using highly charged inorganic main group clusters known as Zintl ions, which are oligomerized in the presence of an organic surfactant using transition metal salts or oxidative coupling. For some materials, * Corresponding author. E-mail:
[email protected]. † University of California Los Angeles. ‡ Lawrence Berkeley National Laboratory. § Current address: Department of Applied Physics, Hanyang University, Ansan, Gyeonggi-do, 426-791, Korea
the organic template can be removed to create a nanoporous semiconductor.32,33 In other cases, the template cannot be removed without destroying the nanometer scale periodicity, but optically or electronically active templates can be employed to create functional heterostructures.34,35 Unfortunately, these Zintl-based semiconductor composites are often formed as powders that have extremely high resistive losses at grain boundary contacts.27 To address this problem, we have developed ways to grow thin films of these composites that reduce the detrimental effects of these resistive boundaries.36 Making conductive mesostructured films may be advantageous in a broad range of applications in which current is injected into or generated by a semiconductor. Now that synthetic methods have been developed, the next requirement for device applications is to understand how semiconducting valence and conduction band levels can be tuned to match the energy levels of relevant electrodes or other chromophores.37-40 Previously, our research on hexagonal honeycomb structured composites made using platinum-coupled SnTe44- clusters showed that valence and conduction band energies can be shifted by changing the relative ratios of the elemental components. This tunability is reminiscent of chalcogenide glass semiconductors (CGSs).27 Bulk CGSs are a class of semiconductors that are composed of main group elements, one or more chalcogenide elements, and, in some cases, transition metal elements. The semiconducting band structure of bulk CGSs have been extensively studied through density of states (DOS) analysis. Previous valence band DOS analysis of chalcogenide semiconductors shows that the valence band edge is formed predominately from chalcogenide orbitals.41-44 It is this unique valence band structure that allows the absolute energy of the CGS valence band to be tuned.44,45 DOS calculations of chalcogenide glass semiconductors have also established that the conduction
10.1021/jp806857v CCC: $40.75 2009 American Chemical Society Published on Web 04/09/2009
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band edge is composed mainly of group IV orbitals, which can shift in energy when dopant metallic elements are incorporated in the CGS.42,46 In this paper, we use these electronic structure concepts to broadly explore how valence and conduction band energies can be tuned in amorphous semiconducting Zintl-cluster-based nanostructured composites. Ultraviolet-visible absorption (UV-vis) and ultraviolet photoelectron spectroscopy (UPS) were used to characterize the electronic structure of semiconducting composite films. We find that the valence and conduction band energies could be predictably tuned over a broad energetic range by selectively altering the metallic coupling agent, the chalcogenide and group IV elements of the Zintl clusters, or the nanoscale architecture. Density of state calculations developed to describe the electronic structure of bulk chalcogenide glass semiconductors are predictive of the band structure trends observed in our nanostructured inorganic/organic composite films. Experimental Synthesis. Cetyltriethylammonium bromide (CTEAB) was used as a structure-directing agent in the solution phase synthesis of these inorganic/organic composite materials. This surfactant was synthesized by reaction of triethylamine (Acros) with 1-bromohexadecane (Acros) in ethanol, as previously reported.23,47 The solid state synthesis of the soluble Zintl cluster anions used to form our semiconducting inorganic framework has been previously reported.27,36 The K4SnS4, K4SnSe4, K4SnTe4, K4Sn4Se10, and K4Ge4Se10 solid state syntheses involved alloying potassium metals with the main group elements at 750 °C in an inert argon atmosphere. All Zintl clusters were solvated in degassed ultrapure water (18 MΩ) and filtered to remove any unreacted material. Finally, the purified clusters were isolated by removing the water using vacuum and mild heating (40 °C). The mesostructured platinum tin telluride and platinum tin selenide composite powders were synthesized using a solution phase methodology. A metal cluster/surfactant solution was made by adding K4SnTe4 or K4Sn4Se10 (0.25 mmol) and CTEAB surfactant (0.48 mmol) to formamide (17.7 mmol) (Alfa Aesar). Next, a solution of cross-linking agent was prepared by dissolving the platinum salt K2PtCl4 (0.2 mmol) (Alfa Aesar) in formamide (17.7 mmol) (Alfa Aesar). The solutions were then heated to 66 °C, and the platinum solution was added to the tin telluride solution while stirring. The temperature of the mixture was maintained for 10 min, during which time an appreciable amount of precipitate formed. The precipitate was transferred to a fritted filter and then rinsed dropwise with at least 20 mL of formamide to remove any residual surfactant. Finally, the mesostructured composites were dried for 14 h under vacuum at less than 30 mTorr to evaporate excess formamide. Semiconducting composite films with hexagonal honeycomb structure or a cubic nanoscale periodicity were synthesized on gold- or platinum-coated glass slides with titanium used as an adhesion layer. The titanium layer was usually 10 nm thick, and the gold or platinum layer was 210 nm thick. The slides were sonicated in alconox-saturated water for 20 min, rinsed with 18 MΩ water, and blown dry with nitrogen prior to use. All syntheses were carried out under inert conditions, which included the use of oxygen-free solvents. The gold-coated slides were inverted and supported in a synthesis solution consisting of Zintl cluster (0.1 mmol), cetyltriethylammonium bromide (0.3 mmol), and transition metal (0.1 mmol) in formamide (1 mL). For the cubic phase, these ratios were adjusted slightly to contain
Korlann et al. Zintl cluster (0.2 mmol), cetyltriethylammonium bromide (0.95 mmol), and transition metal (0.065 mmol) in formamide (1 mL). All of the film mixtures were prepared in an inert atmosphere. The growing films were maintained at 66 °C under inert conditions. Films which employed nickel or palladium to crosslink the Zintl clusters were grown for 25 min; those utilizing platinum salts required approximately 1 h. Different times were used in part because the Pd2+ degraded the gold substrate, so shorter growth times were needed to minimize degradation. The longest growth times were needed for the adamantine tin selenide cluster (Sn4Se104-), which required 1.5 h to form a film having a thickness of approximately 500 nm. Characterization. Film thickness was determined using a Veeco Dektak 6 M profiler. The sample film height was assigned as the difference between the height of the film and bare gold substrate on which the film was grown. Low-angle X-ray diffraction (XRD) patterns were collected using a Panalytical X’Pert Pro powder diffractometer operating with Cu KR radiation. To prevent oxidation during this measurement, samples were impulse-sealed in Mylar under nitrogen. To reduce attenuation of the incident X-ray beam from Mylar adsorption, the path length of the beam through the mylar was minimized by using a thin window (0.5 mil) that was positioned normal to the incident X-rays. Near IR/UV-vis reflectance data were collected using a Shimadzu UV-3100 spectrophotometer. Because of the optical density of the samples, reflectance data were taken utilizing an ISR-3100 integrating sphere attachment. The collection was made over the wavelength range from 200 to 2400 nm. To prevent oxidation during the reflectance measurement, samples were grease-sealed and encapsulated under nitrogen using two borosilicate microscope slides. Elemental composition was measured using energy dispersive spectroscopy (EDS) X-ray analysis, carried out on a JEOL TSM6700F field emission scanning electron microscope (SEM) equipped with a liquid nitrogen-cooled EDAX Super UTW Detector. All elemental emission was obtained with the SEM operating at 25 keV and 20 mA, with the exception of the platinum M emission line, which was collected at 5-6 eV and 20 mA. All spectra were analyzed using EDAX Inc. Genesis Spectrum SEM Quant ZAF Software (Version 3.60). UPS was performed on beam line 9.3.2 at the Advanced Light Source, which is a division of Lawrence Berkeley National Laboratory. Composite films were grounded with an indium wire to the sample holder, which was verified with an ohm meter. In general, sample charging was not observed, which manifests itself as a shifting or broadening of the core level energies in the photoelectron spectrum was not observed. Composite powder samples were mixed in a 1:1 ratio with a carbon black powder, which acts as a solid electrolyte to prevent charging. Next, the sample powder was pressed into indium foil, and a silver paste contact was made at the edge of the sample to ensure good electrical contact to the sample holder. All samples were handled and loaded into the UPS chamber under inert atmosphere. The experiment was run with the UPS chamber under ultrahigh vacuum conditions (∼10-9 Torr). All samples were exposed to incident photons with an energy of 95.0 eV, and the kinetic energies of electrons injected from the valence band was measured. For data analysis, the position of the valence band was determined with respect to a pure gold foil (5.1 eV), which was cleaned by argon ion sputtering at room temperature under UHV conditions.48
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Figure 2. SEM image of a hexagonal inorganic/organic composite film formed from a Sn4Se104- Zintl anion cross-linked with a Pt salt in the presence of an organic surfactant. The image shows the semiconducting film formed on top of and between interdigitated gold electrodes that were patterned onto a silica surface. The slightly darker portions of the SEM image show the Pt-Sn4Se10 film formed on the underlying SiO2 portion of the electrode array. The image shows that the film is continuous and fairly homogeneous across millimeter-scale distances. It does, however, contain random micrometer-sized crescent shaped cracks, which presumably arise from drying strains during solution processing.
Figure 1. (A) Low-angle X-ray diffraction from a sampling of honeycomb structured inorganic/organic composite films that are formed from a SnTe44-, SnSe44-, or Sn4Se104- Zintl anion and cross-liked with either a Ni or Pt salt. Peaks that can be indexed to a p6mm two-dimensional hexagonal phase are observed in the X-ray diffraction for all composites. When nickel is used to cross-link the framework, reaction rates are very fast, and the film is kinetically trapped in a less ordered state, causing an almost complete loss of the higher order (11) and (20) diffraction peaks. By contrast, Pt-coupled Sn4Se104- films form very slowly and show very sharp diffraction peaks indicative of good nanometer scale periodicity. (B) Low-angle X-ray diffraction from mesostructured inorganic/organic composite films and powder formed from a Sn4Se104- Zintl anion and cross-liked with a Pt salt. The (10) fundamental peak and (11) and (20) higher order peaks of a p6mm two-dimensional hexagonal phase are observed in the diffraction for both samples. The Pt-coupled Sn4Se104- films show much sharper peaks when compared to powders, however, indicating that the substrate stabilizes the nanometer scale periodicity.
Results Previously, we have reported that semiconducting chalcogenide-based mesostructured composite films can be formed on conductive gold substrates.36 Profilometry indicated that these thin films can have a range of thicknesses that can vary anywhere form 90 to ∼500 nm, on the basis of the reactivity of the cross-linking metal, the growth time of the film, and the surfactant concentration. X-ray diffraction indicated that these films have the capacity to form with the same hexagonal honeycomb or cubic structures as their counterpart powders.36 XRD patterns in Figure 1A show that semiconducting composite films with many different elemental components are able to form
Figure 3. Tauc fits to near IR/UV-visible diffuse reflectance data for semiconducting inorganic/organic composites made using platinum salts to couple SnTe44-, SnSe44-, or SnS44- clusters. Here ε2, the imaginary part of the dielectric constant, is related to the adsorption coefficient R (R ) ε24π/2nλ). The band gap is assigned to the intercept of the energy axis with a fit to the linear portion of the Tauc curve.49,50 Tauc fits for each composite are shown as thin, solid lines on the graph above. It is evident from the Tauc plot that a wide span of band gaps can be formed by varying elemental composition.
the same p6mm hexagonal honeycomb mesostructure, as indicated by peaks spaced at a ratio of 1:3:2, corresponding to the (10), (11), and (20) diffraction peaks. The long-range periodicity of the mesostructure formed is affected by the reactivity of both the cross-linking metal and the substrate. For example, the (11) and (20) overtones are almost completely absent for nickel cross-linked materials. These composites undergo fast cross-linking that kinetically traps the mesostructure in a less-ordered state. By contrast, extremely narrow diffraction peaks are seen in the slowly grown films formed using platinum to cross-link Sn4Se104- adamantine clusters. Peaks this narrow are not observed in analogous powders, as seen in Figure 1B. The enhanced higher-order peaks suggest that the substrate may stabilize the nanometer scale periodicity of the film. However, evidence presented later shows that the electronic properties of these films closely resemble their powder counterparts, indicating that at the atomic scale, the films are very similar.
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TABLE 1: Summary of Valence and Conduction Band Energies, Band Gaps, and Elemental Compositions for a Range of Surfactant Templated Nanostructured Semiconductor Thin Filmsa phase
composite
valence band (eV)
conduction band (eV)
band gap (eV)
elemental analysis
hex hex hex cubic hex hex hex hex
Pt-SnTe4 Pt-SnSe4 Pt-SnS4 Pt-SnSe4 Pd-Sn4Se10 Ni-Sn4Se10 Pt-Sn4Se10 Pt-Ge4Se10
-5.8 -6.6 -7.9 -6.3 -6.3 -6.3 -5.9 -6.4
-5.2 -5.3 -6.1 -5.2 -4.9 -4.8 -4.4 -4.7
0.55 1.25 1.79 1.10 1.43 1.48 1.46 1.66
Pt1.4SnTe4.2 Pt1.3SnSe4.6 Pt1.3SnS3.8 Pt1.3SnSe4.6 Pd1.4Sn4Se12.7 Ni1.4Sn4Se12.4 Pt1.3Sn4Se9.7 Pt0.8Ge4Se10.4
a For all band gaps, the error is approximately 0.05 eV, whereas the error for valence band values is estimated at 0.06 eV. The composite error for the calculated conduction band energies is 0.08 eV. The nanoscale periodicity of each composite is also listed (under phase) and although only inorganic elemental compositions are shown, all films contain CTEAB surfactant. It is evident that by changing elemental composition, the band gap and valence and conduction band energies of the inorganic network can be tuned. Bold type is used to identify sample groups that vary in only one manner. All energy levels were obtained from near IR/UV-visible diffuse reflectance and UPS. Elemental compositions were determined by EDS X-ray analysis.
A scanning electron microscope (SEM) image of a mesostructured inorganic/organic composite film formed from a Sn4Se104- Zintl anion and cross-linked with a Pt salt is shown in Figure 2. The image shows the semiconducting film formed both on top of and between a series of interdigitated gold electrodes that were lithographically patterned onto a thick silica layer on an intrinsic (undoped) silicon wafer. The slightly darker portions of the SEM image show the Pt-coupled Sn4Se10 film formed on the underlying SiO2 substrate. It is assumed that the film in this region nucleates off the gold fingers and then grows to span the gap. Although the film is quite continuous and homogeneous across many millimeters, it does contain a series of micrometer-scale crescent-shaped cracks. We assume that these cracks form upon drying of the solution processed film. The fact that homogeneous, continuous films can be formed between interdigitated gold electrodes allows us to measure conductivity in these materials. Honeycomb-structured versions of both Pt-coupled Sn4Se104- and SnTe44- composites were formed on these gold electrodes. The conductivity of the Pt-Sn4Se10 film (Figure 2) was found to be 0.000 14 Ω-1 cm-1. Similarly, the conductivity measured for the Pt-SnT4 film was 0.000 36 Ω-1 cm-1. This latter value is quite interesting because it approaches the predicted maximum possible conductivity of 0.0057 Ω-1 cm-1 for these nanostructured films based on AC impedance measurements of intrinsic conductivity in nanostructured powders. Because of the very reasonable agreement between AC and DC measurements, we can say with more confidence that the films are, indeed, electrically homogeneous and that the micrometer-scaled cracks do not have a significant impact on conductivity in this interdigitated electrode geometry. The optical band gaps of our semiconducting composite films were measured using ultraviolet-visible reflection spectroscopy. The relative absorptivity was calculated from the reflectance and is shown in Figure 3, plotted in Tauc form. The band gap is found as the intercept of the linear portion of the curve with the baseline.49,50 We estimate the error in all band gap values to be ∼0.05 eV, a value that is based on errors in our ability to pick the best linear region for regression analysis. The band gaps of the semiconducting nanostructured films formed are summarized in Table 1.27 The band gap energies are seen to increase from 0.55 to 1.25 eV and, finally, to 1.79 eV as the group IV element is substituted from tellurium to selenium and, finally, to sulfur, as seen in Figure 3. The band gaps follow the trends with elemental composition that have been reported previously for nanostructured CGSs.24-27 Although the trends in band gap are well-established and fairly straitforward, the trends in valence and conduction band
energies must also be understood to obtain a complete description of the semiconductor electronic properties.23-32 The valence bands determined through photoelectron spectroscopy are reported in Table 1 for these semiconductors. The valence band energy is assigned as the intercept of the photoelectron intensity with the baseline signal, as seen in Figure 4. All spectra are referenced to the vacuum level (E ) 0 eV) and calibrated to the gold Fermi level, which has a binding energy of 5.1 eV. The conduction band edges reported in Table 1 and depicted in Figures 5-10 were determined by taking the valence band energy and augmenting it, toward the vacuum level, with the energy of the optically determined band gap. We estimate the error in all conduction band values to be ∼0.06 eV, a value that is again based on errors in our ability to pick the best linear region for regression analysis. The possibility of changes in electronic properties arising from the substrate was eliminated by comparing powder and film samples with the same elemental composition. Figure 5 shows data for two different film/powder pairs with very different band gaps. In both cases, the data is identical within the error of the measurements. Thin film samples were used in valence band measurements because they were more conductive and less likely to charge during the UPS measurements when compared to powder composites. We generally find that carbon black powder does not completely permeate powder/composite samples, leaving some highly resistive contacts that prevent measurement on a high fraction of sample. This problem is reduced when films are instead employed. The experimental band properties summarized in Table 1 illustrate that the band energies can be tuned by changing the nature of the chalcogenide element used in the composite. It can be seen in Figure 6 that the valence band energy in our nanostructured semiconductor films shifts from -7.9 eV to higher binding energies of -6.7 and -5.8 eV as the chalcogenide element is changed from sulfur to selenium and then tellurium, respectively. The optical band gap is observed to range from 1.79 to 0.55 eV over this chalcogenide series. Changing the chalcogenide element has less of an effect on the conduction band, which is observed to shift by only 0.9 eV, in contrast to the valence band energy and the optical band gap, which are altered by 2.1 and 1.2 eV, respectively. The semiconducting band properties of these mesostructured composite films can also be altered by changing the group IV element, as seen in Figure 7. When tin was replaced with germanium in the inorganic framework, both the valence and conduction band energies were stabilized and shifted from -5.9 and -4.3 eV to lower binding energies of -6.4 and -4.7 eV,
Electronic Properties of Semiconductor Films
Figure 4. UPS of various honeycomb-structured composite films. The binding energy is shown with reference to the vacuum level (E ) 0). The valence band edge was assigned to the intercept of the baseline with a linear fit to the data. The absolute energy was calibrated using the Fermi energy of gold. The valence band shifts to higher energies as the chalcogenide atomic number is increased, a fact that results from a loss of nuclear stabilization of the lone pair orbitals of heavier chalcogenides.53
Figure 5. A summary of the absolute energy levels observed for nanostructred composite films and powders. It is observed that nucleating the mesostructured composite off a gold surface does not significantly alter the electronic structure when compared to analogous powdered samples. Energy levels were obtained from near IR/ UV-visible diffuse reflectance measurements and ultraviolet photoelectron spectroscopy. The absolute energies of the valence band edge (top of the black box), the conduction band edge (bottom of the white box), and the band gap are labeled on the figure.
respectively. By contrast, the band gap shows less change and is observed to increase only from 1.46 to 1.66 eV when tin is replaced with germanium. The semiconducting band properties of these films can also be tuned by altering the elemental ratios of chalcogenide to group IV element within the formed mesostructure composite structure.27,42 These ratios are sometimes randomly changed by modifying the composite growth kinetics. Transition metal oxidation state can also be used.27 A more controlled way to tune this ratio, however, is by changing the nature of the precursor Zintl cluster. Tetrahedral clusters bring in group IV and chalcogenide atoms in a ratio of 1:4. By contrast, adaman-
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Figure 6. A summary of the absolute energy levels observed for nanostructured composite films as the chalcogenide component is varied from sulfur to selenium and, finally, to tellurium. The valence band is formed primarily from chalcogenide lone pair orbitals, which have less nuclear stabilization as the chalcogenide atomic number increases, causing a shift in the valence band energies toward vacuum.53 The conduction band remains relatively stable, and as a result, the band gap is observed to decrease across this series. All energy levels were obtained from near IR/UV-visible diffuse reflectance and ultraviolet photoelectron spectroscopy. The absolute energies of the valence band edge (top of the black box), the conduction band edge (bottom of the white box), and the band gap are labeled on the figure.
Figure 7. Changes in the absolute energy levels observed for surfactant-templated composite films as the group IV element is changed from tin to germanium. The valence and conduction bands are both influenced by the group IV element and exhibit a shift to lower absolute energies when germanium was substituted for tin. All energy levels were obtained from near IR/UV-visible diffuse reflectance and ultraviolet photoelectron spectroscopy. The absolute energies of the valence band edge (top of the black box), the conduction band edge (bottom of the white box), and the band gap are labeled on the figure.
tine clusters contain 4 group IV atoms and 10 chalcogenides for an atomic ratio of 1:2.5. The elemental analysis shown in Table 1 indicates that these elemental ratios are not significantly altered upon formation of the mesostructure. Therefore, large changes in band energy can be observed using the precursor Zintl cluster to tune elemental ratios, as seen in Figure 8. When the composite was synthesized with an adamantine cluster, the concentration of the group IV atoms was increased, correlating
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Figure 8. Changes in the absolute energy levels observed for semiconducting composite films formed using either a tetrahedral SnSe44- or an adamantine Sn4Se104- Zintl cluster. When the adamantine Sn4Se104- cluster is substituted for the tetrahedral SnSe44- cluster, the electropositive tin concentration increases and destabilizes the valence band, causing a shift toward the vacuum energy level. The platinum/ tin ratio also decreased, which removes metallic states at the conduction band edge and causes a shift to higher energy. This results in large changes in absolute energy levels with minimal changes in the band gap. All energy levels were obtained from near IR/UV-visible diffuse reflectance and UPS. The absolute energies of the valence band edge (top of the black box), the conduction band edge (bottom of the white box), and the band gap are labeled on the figure.
with valence and conduction band shifts from -6.6 and -5.3 eV for the tetrahedral cluster toward vacuum to energies of -5.9 and -4.4 eV, respectively. By contrast, the band gap changed only slightly, increasing from 1.25 to 1.46 eV for the tetrahedral and adamantine composites, respectively. Changing the nature of the transition metal employed is somewhat more complex, because different metals have different preferred bonding geometries, which can result in altered elemental ratios. For materials in which the elemental composition is not perturbed, the choice of metallic salt that condenses the composite framework is not observed to have a significant affect on either the conduction or valence band energies of these honeycomb-structured semiconductors, as seen in Figure 9. When the metallic salt is the only elemental change in the composite, as observed when replacing nickel with palladium, the valence band remains constant at -6.3 eV, and the conduction band shifts by only 0.1 eV. The valence and conduction band energies do, however, respond to changes in the tin and selenium concentration. Valence and conduction band shifts from -6.3 and -4.9 eV to -5.9 and -4.4 eV were observed when palladium was replaced with the less coordinating platinum ion. Band gaps of the composites shown in Figure 9 were not significantly affected by the metallic component incorporated, regardless of other band energy changes. Finally, the elemental ratios of chalcogenide to group (IV) element can also be altered by making different nanoscale periodicities. The root of this effect probably lies in the fact that each phase with a different inorganic/organic interface has a different optimal interfacial charge density. The elemental analysis shown in Table 1 indicates that the elemental ratios of chalcogenide to group IV decreased from 4.6 to 3.7 in moving from a hexagonal to a cubic nanoscale periodicity. This gives rise to a shift in the valence band from -6.6 eV to a higher binding energy of -6.3 eV, as seen in Figure 10.
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Figure 9. A summary of the absolute energy levels observed for nanostructured composite films as the metallic element is varied. Metals that exhibit octahedral bonding (Ni and Pd) show similar data, whereas square planar metals such as Pt show different band energies. Shifts in the conduction band are assigned to intrinsic differences in the energies of the metal orbitals in the two geometries. Valence band shifts, by contrast, appear to arise from the increased Se content needed to satisfy the higher coordination of Ni and Pd. All energy levels were obtained from near IR/UV-visible diffuse reflectance and UPS. The absolute energies of the valence band edge (top of the black box), the conduction band edge (bottom of the white box), and the band gap are labeled on the figure.
Figure 10. Changes in the absolute energy levels for nanostructured composites films formed with either a hexagonal honeycomb geometry or a cubic structure. The higher-curvature cubic phase requires a lower cationic surfactant density and, thus less anionic Se to maintain charge neutrality. In the cubic phase, the relative concentration of electropositive tin increases as compared to selenium and destabilizes the valence band, causing a shift toward the vacuum level. All energy levels were obtained from near IR/UV-visible diffuse reflectance and UPS. The absolute energies of the valence band edge (top of the black box), the conduction band edge (bottom of the white box), and the band gap are labeled on the figure.
Discussion As seen for bulk chalcogenide glass semiconductors, our nanostructured films are found to exhibit shifts in valence and conduction band energies that are attributed to changes in the atomic makeup of theses bands. Both the valence and conduction band energies can be tuned by ∼2 eV through manipulation of
Electronic Properties of Semiconductor Films the elemental composition in our honeycomb-structured inorganic/ organic composite films, as shown in Table 1. The electronic properties of chalcogenide-based nanostructured semiconductors can be explained by relating to band theory developed to describe the electronic structure of chalcogenide glass semiconductors. It has been observed in DOS calculations for PbTiO3, SnTe, and TiX2 (X ) S, Se, Te) that the valence band edge is composed mostly of nonbonding chalcogenide p-orbitals.42,51,52 Our nanocomposite valence bands should be similar to these semiconductors because the inorganic framework is formed from a mixture of transition metals, group IV elements, and chalcogenides. To begin this analogy, we examine materials in which the chalcogenide element is changed from sulfur to selenium and then to tellurium. Evidence that the chalcogenide orbitals play a dominant role in forming the valence band in our composite films is found in the 2.1 eV displacement of the valence band observed when the chalcogenide component is varied, as seen in Table 1 and Figure 6. The explanation for this displacement, by analogy to bulk CGSs, relates to the fact that the nonbonding chalcogenide p-orbitals that make up the valence band behave similarly to free chalcogenide atomic orbitals. These orbitals are energetically destabilize in heavier elements as the distance from the nucleus increases.44,53 Changing the chalcogenide element has less of an effect on the conduction band, which is observed to remain fairly stable, relative to the valence, band across the series. As a result, the optical band gap decreases drastically when varying the chalcogenide elements from sulfur toward tellurium. For bulk CGSs, it has been observed that all band energies are influenced by the nature of the group IV element.42,54 The valence band, formed from chalcogenide lone pair p-orbitals, becomes destabilized by neighboring electropositive group IV elements.44,54 In addition, the conduction band in bulk chalcogenide semiconductors is predominantly formed from group IV p-orbitals, as seen for DOS calculations.42 In agreement with this idea, when the group IV element within our nanostructured films is varied from tin to germanium, all semiconducting band energies shift. When germanium replaces tin, the valence band is stabilized. This is expected, because germanium is less electropositive than tin.55 However, the observed valence band energetic shift of 0.5 eV (Figure 7) is larger than might be anticipated for the slight difference in electropositivity between germanium and tin. One possible explanation for this large shift is that the tin orbitals are spatially more diffuse than germanium, thus bringing the electropositive orbitals closer to the selenide lone pair orbitals and causing increased interaction. Similarly, the conduction band energy is observed to shift by 0.4 eV in our films when moving from tin to germanium, as seen in Figure 7. This agrees with CGS predictions that the group (IV) element is a major component of the conduction band. Because of these complimentary valence and conduction band shifts, the band gap is observed to increase only slightly as tin is substituted with germanium, consistent with previously published results.25 The discussion above focuses on how substituting elements within our nanostructured semiconducting films affected the band structure. Previously, we have shown that elemental ratios in Pt-coupled SnTe44- mesostructured composites could be changed by varying the oxidation state of the platinum precursor used for cross-linking, and those elemental ratio changes can also be used to tune the valence and conduction band energies.36 Here, we demonstrate similar, but more controlled, changes in atomic ratios by forming films with either a tetrahedral SnSe44or an adamantine Sn4Se104- Zintl cluster. Because the clusters have the same charge, this substitution results in changes in
J. Phys. Chem. C, Vol. 113, No. 18, 2009 7703 both the chalcogenide to tin and platinum-to-tin ratios, as seen in Figure 8. When the adamantine Sn4Se104- cluster is substituted for the tetrahedral SnSe44- cluster, the electropositive tin concentration increases and destabilizes the valence band, causing a shift toward the vacuum energy level. At the same time, to maintain charge neutrality, the platinum-to-tin ratio is reduced from 1.3 to 0.3 in films formed from the adamantine clusters. In bulk CGSs, the conduction band has been observed to shift when the metallic component is perturbed. For example, in Se-Te-As-Ge materials, the addition of metal atoms adds states at the conduction band edge, which effectively lowers its energy while having no effect on the valence band energy.56 By analogy, we expect the reduced Pt content in materials formed from the adamantine cages to remove metallic states at the conduction band edge and cause the conduction band to shift toward the vacuum energy level. This is, indeed, observed.27 Finally, the band gap is observed to increase slightly because the conduction band shift is a bit larger than the valence band shift. To a first approximation, however, the band gap is nearly constant, whereas the valence and conduction bands show large shifts. We next consider whether the conduction band energy depends on the nature of the metallic component by comparing nanostructured films made with Ni, Pd, and Pt. For Ni- and Pd-coupled materials, the data are basically identical, indicating that the nature of the metal, as opposed to the amount, is unimportant (Figure 9).56 The data for both of these metals are different from that obtained for Pt coupled materials, however. Careful examination of the elemental ratios presented in Figure 9 indicates that both the Ni and Pd films contain extra Se. This is likely because both of these metals tend to form octahedral complexes in these types of materials, but Pt forms only square planar complexes.23,57 We thus postulate that the conduction band shifts stem from this change in transition metal coordination, which in turn modifies the nature and energy of the transition metal orbitals that can mix with the conduction band states. A similar change in valence band energies is also observed (Figure 9), and here, we postulate that the change does not stem directly from modification of the transition metal orbitals,56 but instead comes from the added chalcogenide and, thus, the increased chalcogenide/group IV ratio. Because the valence and conduction band shifts are so well matched, the nature of the transition metal has little effect on the band gap. Finally, we observe that the elemental composition and, thus, the electronic structure of these materials can be perturbed by altering the nanoscale architecture. Changes in elemental composition arise because different architectures (hexagonal versus cubic) have different optimal interfacial charge densities, and thus a different optimal cation-to-anion ratio.58 Specifically, most cubic phases have higher interfacial curvatures and, thus lower interfacial charge densities than hexagonal honeycomb phases.59 Because the interface must be anionic to associate with the cationic surfactant, the reduction in charge density translates to a reduction in the Se content. We found that the chalcogenideto-group IV ratio went from 4.6 to 3.7 when changing the mesostructure from hexagonal to a cubic, as seen in Figure 10. In the cubic phase, the decrease in Se and, thus, the relative increase in Sn destabilizes the valence band and raises its energy. Unlike earlier examples, however, in which the Sn/Se ratio changes were accompanied by changes in Pt concentration to maintain charge neutrality, for these materials with different nanometer scale architectures, different amounts of cationic surfactant preserve charge neutrality. As a result, the Pt/Sn ratio
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does not change, and so the conduction band remains relatively stable. This ultimately results in a decrease in the band gap. Conclusion In this work, we have shown that the elemental compositions of surfactant-templated nanostructured semiconductor films can be altered to tune the band gap and the valence and conduction band energies. For example, the valence band can be tuned over 2.1 eV by changing the chalcogenide element, whereas the conduction band energy showed a much smaller change (0.9 eV). Changing either the nature of the group IV element, the chalcogenide/group IV ratio, or the bonding geometry of the transition metal shift both the valence and conduction band energies in the same direction. These concurrent changes are caused by the destabilization of the chalcogenide-dominated valence band as the concentration or electropositive nature of the group (IV) element is increased. At the same time, as the group IV concentration increases, the condition band energy also shifts toward the vacuum level. Shifts in the conduction band can also stem from the transition metal. Octahedral metals appear to more effectively add states at the conduction band edge than square planar metals. The ability to tune the valence and conduction band energy while keeping the band gap constant or, alternatively, to tune the valence band energy and band gap while keeping the conduction band energy fairly constant is extremely useful. These templated nanostructured frameworks thus hold several advantages for the design and synthesis of devices. Films can be selectively deposited through solution phase routes using the chalcogenide affinity to bind to gold. Furthermore, the ability to control the elemental compositions of the nanostructured films allows the band structure of the inorganic framework to be tailored for specific applications. Current research is underway to create composite materials using an organic semiconductor as the structure directing agent.34,35 Such materials would make good candidates for device applications such as photovoltaics. Moreover, it is likely that these same band energy trends will hold for nontemplated versions of chalcogenide glass semiconductors synthesized using Zintl cluster precursors.60-62 As a result, the data presented here provide a basis to predicatively synthesis a broad range of semiconductors with desired band properties using Zintl cluster precursors and simple solution phase methods. Acknowledgment. UPS data were collected at the Advanced Light Sources (ALS), which is operated by the Department of Energy, Office of Basic Energy Science. This work was supported by the American Chemical Society Petroleum Research Fund under Grant ACS PRF# 46107-AC5, by National Science Foundation under Grant CHE-0527015, and by the Office of Naval Research under Grant N00014-04-1-0410. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartulli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Huo, Q.; Margolese, D. I.; Clesa, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schuth, F.; Stucky, G. D. Nature 1994, 368, 317. (4) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger, P.; Hue, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. (5) Gross, A. F.; Ruiz, E. J.; Tolbert, S. H. J. Phys. Chem. B 2000, 104, 5448.
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