Synthesis of freestanding HfO2 nanostructures
© Kidd et al; licensee Springer. 2011
Received: 30 October 2010
Accepted: 5 April 2011
Published: 5 April 2011
Two new methods for synthesizing nanostructured HfO2 have been developed. The first method entails exposing HfTe2 powders to air. This simple process resulted in the formation of nanometer scale crystallites of HfO2. The second method involved a two-step heating process by which macroscopic, freestanding nanosheets of HfO2 were formed as a byproduct during the synthesis of HfTe2. These highly two-dimensional sheets had side lengths measuring up to several millimeters and were stable enough to be manipulated with tweezers and other instruments. The thickness of the sheets ranged from a few to a few hundred nanometers. The thinnest sheets appeared transparent when viewed in a scanning electron microscope. It was found that the presence of Mn enhanced the formation of HfO2 by exposure to ambient conditions and was necessary for the formation of the large scale nanosheets. These results present new routes to create freestanding nanostructured hafnium dioxide.
PACS: 81.07.-b, 61.46.Hk, 68.37.Hk.
Owing to its high dielectric constant and lack of reactivity with silicon, hafnium dioxide has excellent characteristics for replacing SiO2 in nanometer scale applications such as gate oxides [1, 2]. In addition to applications in electronics as thin films, there have been reports of interesting properties of HfO2 when synthesized in the form of nanocrystals or nanorods [3–5]. Inducing dimensional constraints by reducing the size of one or more dimensions has produced emergent phenomena in a range of materials such as graphene [6, 7], single layer dichalcogenides , and other two-dimensional systems . An example for the HfO2 system was that defect concentrations are easier to control when the HfO2 is formed as nanorods . These defects can induce ferromagnetism, which has been far more difficult to reproduce in macroscopic HfO2.
With regards to nanostructure synthesis, the creation of two-dimensional freestanding nanostructures is of special interest. Most device applications entail the use of materials in the form of thin films. Determining the intrinsic properties of such films is difficult. Properties of the interfaces between the film and other components of the device can obscure the intrinsic properties of the film, and the interfacial effects only become larger as film thickness is decreased to nanometer scale dimensions. This issue has in part led to the development of synthesis techniques for creating various materials as freestanding, two-dimensional nanostructures [8–11].
In this work, we report two new methods for creating nanostructured HfO2. We have synthesized nano-scale crystallites of HfO2 as well as highly two-dimensional freestanding HfO2 nanosheets as a byproduct of the synthesis of HfTe2. The nano-scale crystallites were formed as a natural decomposition product from exposing HfTe2 to ambient conditions. The freestanding, two-dimensional oxide structures were induced to grow using a slightly modified growth process that normally yields HfTe2 in powder form. Both processes are extremely simple and represent new routes for synthesizing nanostructured HfO2 that could lead to new routes for inducing dimensional constraints in this material. Furthermore, as the HfO2 nanocrystallites are formed from the decomposition of powdered HfTe2, which is a layered material, it is expected that these structures are highly two-dimensional as well.
A mixture of HfTe2 and HfO2 was synthesized using standard techniques for growing transition metal dichalcogenides. Stoichiometric amounts of Hf and Te powders (Alfa Aesar, >99% purity) were added to a fused silica ampoule that was typically 8 cm long with a 1.1 cm inner diameter. The ampoules were then sealed under vacuum at a pressure of less than 0.1 mTorr. Samples were first heated to 125°C for 24 h to ensure that the ampoules would not burst from over-pressurization due to tellurium. The annealing temperature was then raised to 900°C and held at this temperature for several days. After the ampoules were opened, it was found that HfTe2 readily decomposed into HfO2 when exposed to ambient conditions. In most cases, it appeared that the original product was a powder consisting entirely of HfTe2, with HfO2 forming as a decomposition product after the ampoules were opened. Several attempts were also made to incorporate Mn or Cr dopants into the HfTe2 crystals. Doping levels up to a nominal 25% incorporation (i.e., Mn0.25HfTe2) were attempted for both elements. Powders of these elements (Alfa Aesar, >99.9% purity) would be mixed in various amounts with the original Hf and Te powders before the ampoules were sealed.
Sample products were measured using X-ray diffraction (XRD) with a Rigaku MiniFlex II. XRD measurements were performed on a silicon zero background sample holder for both powdered specimens and macroscopic HfO2 sheets. Powdered specimens were sifted through a -200 mesh (75 μm) sieve while larger sheets were laid flat upon the sample holder. X-ray analysis was performed using CrystalMaker™ software. The structural properties were measured using an Everhart-Thornley detector in a Tescan Vega II scanning electron microscope (SEM). Energy dispersive X-ray spectroscopy (EDS) was performed using a Bruker Quantax 400 system attached to the SEM. The images and EDS analysis shown here were performed using 20 kV electrons. Samples were fixed to aluminum posts for SEM measurements using double-sided carbon tape. Larger sheets were sufficiently stable for manipulation using tweezers and other instruments. Smaller powders were sifted onto the carbon tape for measurement.
Results and discussion
The formation of HfO2 was actually an unintended consequence from attempts to grow pure and doped crystals of HfTe2. The actual products were a mixture of HfTe2 powders in the form of sub-millimeter crystals and products consisting of HfO2. It was also found that HfTe2 decomposed rather quickly into HfO2 upon exposure to air. The dopants, Mn or Cr, were never successfully incorporated into the main products, forming either impurity phases or ending up as a metallic residue on the walls of the ampoule. However, the inclusion of Mn did enhance the formation of HfO2 both during synthesis and after the samples were exposed to air.
It is not clear why the addition of Mn enhanced the formation of HfO2. Oxygen impurities in dichalcogenides have been reported in samples grown with manganese due to the manganese oxide which can readily form on powder Mn . These samples also contained a larger than usual amount of MnTe impurity phase, thus reducing the overall amount of Te available for reaction and possibly inducing the Hf to scavenge small amounts of oxygen from the interior walls of the ampoules. After the ampoules were opened, the HfTe2 powders which contained Mn also converted to HfO2 more quickly, indicating the Mn might act as a catalyst for the oxidation reaction. This could also explain the enhanced formation of sheets within ampoules containing Mn. It is more likely that HfTe2, a relatively unstable compound, would be formed as an intermediate step before oxidation into HfO2 during the crystal growth rather than pure Hf scavenging oxygen its environment.
The HfO2 nanosheets were extremely thin considering their surface area, which ranged up to 25 mm2. These structures could be picked up with tweezers or otherwise manipulated for study by SEM, although some breakage and tearing occurred during handling. While somewhat brittle in their sensitivity to manipulation, the sheets were otherwise stable even after being studied for several months. The sheets showed signs of charging in the SEM, but not as much as might be expected from a wide gap insulator. As might be expected for a charging sample, edges of the sheet viewed at high magnification would tend to vibrate and wobble. This effect could be reduced by lowering the beam current and/or magnification. Bright and dark fringe patterns commonly seen on highly insulating materials like silica were not found, however. This indicates that the sheets behave more like semi-conducting materials than true insulators. This behavior is consistent with the presence of defects in the crystal lattice that would add carriers or reduce the band gap as has been seen in other examples of nanostructured HfO2 .
Another interesting feature common to both sides was the existence of small dark circles visible in Figure 2c. The size and spacing of these features was the same on both sides, indicating that they are likely pores in the structure. Measurements taken on the darker side, which were easier to focus on, showed that these features were all about 100 nm in diameter and surrounded by rings that were relatively bright compared to the rest of the surface. These dark spots were irregularly spaced but very consistent sizes, varying by less than 20%. While their origin is unclear, these features could arise from defect clusters induced by the high degree of anisotropy of the sheets. It is also possible that they could arise from crystal strain induced by a chemical reaction transforming hexagonal HfTe2 into monoclinic HfO2.
The image of Figure 4 was taken using 20 kV electrons which have a mean free path of approximately 10 nm in most materials . The secondary electrons measured in this image typically have energies less than 50 eV which have mean free paths on the order of 1 nm. To be imaged through the upper sheet, the electron beam had to pass through the sheet and create secondary electrons on the surface of the bundle. These secondary electrons would then need to pass through the sheet again to reach the detector. This could only occur if the sheet thickness was not more than a few nanometers, implying the entire structure was only several molecules thick. This represents an extremely large anisotropy, as this particular sheet was rectangular with sides measuring roughly 150 μm × 300 μm.
Freestanding two-dimensional nanosheets of HfO2 and nanometer scale HfO2 crystallites were synthesized as byproducts of the attempted growth of pure and doped HfTe2. The oxide growth was enhanced by the presence of Mn in the growth ampoule in both cases. It appears as if the HfO2 sheets were formed during the growth process while the nanometer scale crystallites formed after the ampoules were cracked open and the resulting HfTe2 powders were exposed to air. While it is not clear exactly what form the nanometer scale HfO2 crystallites have, it would not be surprising if they were two-dimensional as well given that their precursor, HfTe2, is itself a highly two-dimensional layered material. Given that it is possible to exfoliate dichalcogenides to create single molecular layers , this synthesis route could be able to yield two-dimensional nanostructures in any case.
The HfO2 sheets were extremely two-dimensional with thicknesses ranging from a few nanometers to no more than a few hundred nanometers. In addition to being extremely thin for their size, they also contained a large number of defects in the form of sub-micron scale holes. It is not clear what effect these structures have, but they could relate to other vacancy type defects that have been shown to influence magnetic behaviors in nanostructured HfO2. These results represent a new route for synthesizing nanostructured HfO2 and the first reported example of freestanding two-dimensional HfO2 nanostructures.
energy dispersive X-ray spectroscopy
scanning electron microscope
This research was supported by the Battelle foundation and the Iowa Office of Energy Independence grant #09-IPF-11. The Rigaku X-ray diffractometer and Bruker EDX systems were purchased by Army Research Office DOD Grant # W911NF-06-1-0484. Dr. Kidd also acknowledges support from a UNI Summer Fellowship.
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