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Growth and Field Emission Study of Molybdenum Oxide Nanostars Ali Khademi,† Rouhollah Azimirad,‡ Ali Asghar Zavarian,§ and Alireza Z. Moshfegh*,†,| Department of Physics, Sharif UniVersity of Technology, P.O. Box 11155-9161, Tehran, Iran, Institute of Physics, Malek-Ashtar UniVersity of Technology, Tehran, Iran, Academic Center for Education, Culture & Research (ACECR)sSharif UniVersity BranchsHigh Vacuum Technology R&D Center, P.O. Box 13445-686, Tehran, Iran, and Institute for Nanoscience and Nanotechnology, Sharif UniVersity of Technology, P.O. Box 14588-89694, Tehran, Iran ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: September 1, 2009
The field emission properties of MoO2 nanostars grown on a silicon substrate and their emission performance in various vacuum gaps are reported in this article. A new structure of molybdenum oxides, named a nanostar, is grown by thermal vapor deposition with a length of ∼1 µm, a thickness of ∼50 nm, and its width in the range of 500-700 nm. The morphology, structure, composition, and chemical states of the prepared nanostars were characterized by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). According to XRD analysis, the grown nanostructures are composed of both crystalline Mo4O11 and crystalline MoO2 structures. XPS analysis showed that the synthesized nanostructures contained ∼21.2% Mo6+, ∼16.2% Mo5+, ∼39.8% Mo4+, and ∼22.8% Moδ+ (where 0 < δ < 4). TEM observations indicate that the synthesized sample consists of MoO2 nanostars over a crystalline thin film containing Mo4O11 nanoparticles. The turn-on emission field and the enhancement factor of nanostars are found to be 1.0 V/µm and 19 070 at the vacuum gap of 500 µm, respectively. These excellent emission properties are attributed to the special structure of the nanostars. Therefore, these nanostars can be used in vacuum microelectronic applications. 1. Introduction Great research interest has been focused on quasi-onedimensional materials due to their unique electronic, optical, and mechanical properties, and as a result, they have promising applications in nanodevices.1 Among many novel properties of nanomaterials, field emission (FE) based on various onedimensional nanostructures, such as carbon nanotubes,2,3 ZnO nanowires,4 tungsten oxide nanowires,5 molybdenum and molybdenum oxide nanowires,6-8 MoO3 nanobelts,9 MoO2 nanorods,10,11 pinaster-like MoO2 nanoarrays,12 MoO3 nanoflowers,13 etc., has attracted prime interest in the past few years due to its potential usages in several vacuum microelectronic applications, such as field emission displays, microwave sources, FE triodes, and amplifiers.14 One-dimensional nanomaterials with a high aspect ratio and small tip radius of curvature are expected to possess a good field electron emission property.8 Local field enhancement at the apex of each nanoscale protrusion of these emitters leads to lowering the turn-on and threshold voltage for field emission.14 In contrast to nanowires and nanotubes having a cylindrical geometry, nanostars have sharp corners and edges. Therefore, it is expected that nanostars possess relatively high field emission properties. It is established that many one-dimensional nanomaterials are oxide semiconductors. Oxide materials, as chemically inert, robust materials that have a relatively high melting temperature, * To whom correspondence should be addressed. E-mail: moshfegh@ sharif.edu. † Department of Physics, Sharif University of Technology. ‡ Malek-Ashtar University of Technology. § (ACECR)sSharif University BranchsHigh Vacuum Technology R&D Center. | Institute for Nanoscience and Nanotechnology, Sharif University of Technology.
are very stable in ambient and low-vacuum conditions, that is, 10-4-10-5 Torr.14 Potential low electron affinity of wide band gap semiconductors is helpful to produce low field emission (high emission in low electric field).8 Among oxide semiconductor emitters, molybdenum oxides are widely used in various applications. Molybdenum has several common oxidation states, namely, +2, +3, +4, +5, and +6, but its oxides mainly include MoO2 and MoO3. Molybdenum oxides are n-type semiconductors with a band gap in the range of 2.8-3.6 eV11,15-17 and a work function in the range of 5.3-6.5 eV,18-21 which makes them attractive for many advanced applications as catalysts, sensors, photochromic and electrochromic materials, recording materials, and field emitters.7,8,10 In the present study, a new type of molybdenum oxide nanostructures (MoO2 nanostars) is grown perpendicularly to the silicon substrate using a thermal evaporation technique. The grown nanostructures were characterized by various analytical techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). Concerning applications, field emission measurements with different vacuum gaps are carried out for the synthesized samples under high-vacuum conditions. 2. Experimental Details 2.1. Synthesis of Nanostructures. Mirror-polished silicon (100) n-type substrates with a dimension of 10 mm × 10 mm were first washed with acetone and alcohol in an ultrasonic cleaner. The substrates and the Mo boat were then placed in a vacuum chamber evacuated by using a rotary pump, in the center of which the Mo boat as evaporation source was heated by electric current. A gas inlet control system and a pyrometer for monitoring temperature were utilized in the chamber. The
10.1021/jp9056237 CCC: $40.75 2009 American Chemical Society Published on Web 10/09/2009
Field Emission Study of Molybdenum Oxide Nanostars
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Figure 1. Low- and high-magnification SEM images of MoOx nanostructures.
substrates were separated from the boat with a spacing of 0.8 mm of alumina strip holders. After evacuation of the chamber to less than 4 × 10-2 Torr, high-purity argon (Ar) gas (99.999%) was introduced into the system at a flow rate of 200 sccm. The temperature of the Mo boat was then ramped to ∼1300 °C in 8.5 min and monitored and estimated by the pyrometer. The boat and the substrates were maintained at the desired temperature and Ar flow at a pressure of 6.5 × 10-1 Torr dynamically introduced for 15 min before the boat was cooled down to room temperature gradually. A similar growth procedure was also used for synthesizing different molybdenum oxide and tungsten oxide nanostructures under low-vacuum conditions previously.7,22,23 2.2. Structure and Surface Analysis. The morphology of the synthesized samples was examined by a JEOL JSM-5910LV SEM. The surface atomic concentration and chemical composition of the samples were studied by XPS equipped with an Al KR X-ray source at an energy of 1486.6 eV. All binding energy values were calibrated by fixing the C 1s line to 285.0 eV (more details can be found in the Supporting Information). A Philips PW3710 profile X-ray diffractometer with a Cu KR radiation source (λ ) 1.54 Å) under the accelerating voltage of 40 kV and the current of 40 mA with a normal θ-2θ scan and step size of 0.05° was used to determine phase formation, crystalline size, and structure of the layers. Detailed structural characterization of the as-grown product was observed by a JEOL 2010 HRTEM using an electron microscope operating at 200 kV (λ ) 0.0251 Å). The molybdenum oxide nanostructures were scratched from the substrate and used on a Cu grid for TEM observation. Field emission measurements were performed in an indigenously fabricated setup at a pressure of 5 × 10-8 Torr. A cylindrical aluminum probe with a diameter of 5 mm was used as an anode, and the synthesized molybdenum oxide nanostructure film served as a cathode (more details can be found in the Supporting Information). 3. Results and Discussion Figure 1a shows a low-magnification SEM image of the synthesized molybdenum oxide nanostar structures. Panels b and c of Figure 1 are higher-magnification SEM images from
Figure 2. High-resolution XPS spectrum of the Mo(3d) peak.
some areas in Figure 1a. Figure 1d is a cross-sectional view of the sample. As displayed in Figure 1, each nanostar is composed of a few nanowalls crossing each other. These nanowalls with length of ∼1 µm grew uniformly and crossingly on the substrate over a large area. The nanowalls have widths in a range from 500 to 700 nm and a thickness of ∼50 nm. A typical widthto-thickness ratio exceeds 10. The nanowalls are straight and have rectangular flat tips with four sharp corners at the upper ends. Smooth facets enclose the surfaces of the nanowalls and form the sharp edges. The nanowalls cross together and form nanostar shapes. As can be seen from a cross-sectional SEM image (Figure 1d), there is a 500 nm thin film composed of nanoparticles with a size of around 100 nm (indicated by the arrow) beneath the nanostars. Molybdenum, oxygen, adventitious carbon, and a trace amount of silicon from the substrate on the surface were detected in the XPS survey spectrum of the sample without other additional elements. Figure 2 illustrates a high-resolution XPS spectrum of the Mo 3d photoemission signal. The oxidation state of various Mo species calculated from the data shown in the high-resolution spectrum of Figure 2 is summarized in Table S1 in the Supporting Information. Upon deconvolution and curve fitting, the XPS plot contains eight bands, namely, Moδ+ 3d5/2, Mo4+ 3d5/2, Mo5+ 3d5/2, Mo6+ 3d5/2, Moδ+ 3d3/2, Mo4+ 3d3/2, Mo5+ 3d3/2, and Mo6+ 3d3/2. The obtained binding energy and spin-orbit splitting (∆ Mo 3d ) 3.15 eV) values listed in Table S1 in the Supporting Information are in good agreement with
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Figure 3. Variation of Mo 3d5/2 binding energies versus their oxidation number.
Figure 4. (a) XRD spectrum of the molybdenum oxide nanostars. (b) The enlargement view of the main peaks of the MoO2 phase as identified in (a).
the value reported for mixed valence MoOx by other groups showing spin-orbit splitting (∆ Mo 3d) values between 3 and 3.2.24-27 It is clear that the area ratio and fwhm ratio of Mon+ 3d5/2 to Mon+ 3d3/2, where n is δ, 4, 5, and 6, should be 3/2 and 1, respectively. The results obviously show that different molybdenum oxides are formed. The valence state ranges from Moδ+ (0 < δ
