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 co- organization 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 transi- tion metal oxides, such as Al 2 O 3 , Mn x O y , Nb 2 O 5 , SnO 2 , TiO 2 , TiO 2 /VO 2 , ZrO 2 , and NiO 2 . 7-20 Although some low-valent Mn and Nb/Ti oxide mesostructures are conducting, 21,22 most oxide- based 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 com- posites 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, 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 SnTe 4 4- 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 semiconduc- tors (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 semicon- ductors shows that the valence band edge is formed predomi- nately 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 * Corresponding author. E-mail: tolbert@chem.ucla.edu. † University of California Los Angeles. ‡ Lawrence Berkeley National Laboratory. § Current address: Department of Applied Physics, Hanyang University, Ansan, Gyeonggi-do, 426-791, Korea J. Phys. Chem. C 2009, 113, 7697–7705 7697 10.1021/jp806857v CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009