Nano Research 1 DOINano 10.1007/s12274Res -014-0567-z Laser induced oxidation and optical properties of stoichiometric and non-stoichiometric Bi2Te3 nanoplates Rui He1 (), Sukrit Sucharitakul2, Zhipeng Ye1, Courtney Keiser1, Tim E. Kidd1, and Xuan P. A. Gao2 Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0567-z http://www.thenanoresearch.com on Aughst 19, 2014 © Tsinghua University Press 2014 Just Accepted This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication. Laser induced oxidation and optical properties of stoichiometric and non-stoichiometric Bi2Te3 nanoplates Rui He1,*, Sukrit Sucharitakul2, Zhipeng Ye1, Courtney Keiser1, Tim E. Kidd1, and Xuan P. A. Gao2 1Department of Physics, University of Northern Iowa, Cedar Falls, Iowa 50614, United States 2Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, United States Bi2Te3 nanoplates are oxidized in the air under heating with moderate laser power. Further increase of laser power burns holes on the sample surface. Optical absorption coefficient in thin Bi-Te NPs strongly depends on their stoichiometry. Rui He, http://www.physics.uni.edu/rui-he Tim E. Kidd: http://www.physics.uni.edu/faculty/tim-kidd Xuan P. A. Gao, http://www.phys.cwru.edu/faculty/?gao Nano Research DOI (automatically inserted by the publisher) Review Article/Research Article Please choose one Laser induced oxidation and optical properties of stoichiometric and non-stoichiometric Bi2Te3 nanoplates Rui He1 (), Sukrit Sucharitakul2, Zhipeng Ye1, Courtney Keiser1, Tim E. Kidd1, and Xuan P. A. Gao2 Received: day month year ABSTRACT Revised: day month year Bi-Te nanoplates (NPs) grown by low pressure vapor transport method were Accepted: day month year studied by Raman spectroscopy, atomic force microscopy (AFM), (automatically inserted by energy-dispersive X-ray spectroscopy (EDS), and Auger electron spectroscopy the publisher) (AES). We find that the surface of relatively thick (more than tens of nanometers) Bi2Te3 NPs is oxidized in the air and forms a bump under heating © Tsinghua University Press with moderate laser power, as revealed by the emergence of Raman lines and Springer-Verlag Berlin characteristic of Bi2O3 and TeO2 and characterization by AFM and EDS. Further Heidelberg 2014 increase of laser power burns holes on the surface of the NPs. Thin (thicknesses less than 20 nm) NPs with stoichiometry different from Bi2Te3 were also KEYWORDS studied. Raman lines from non-stoichiometric NPs are different from those of stoichiometric ones and display characteristic changes with the increase of Bi Bismuth telluride, concentration. Thin NPs with the same thickness but different stoichiometries nanoplate, Raman, show different color contrast compared to the substrate in the optical image. oxidation, stoichiometry This indicates that the optical absorption coefficient in thin Bi-Te NPs strongly depends on their stoichiometry. 1 Introduction single Dirac cone on the surface [1-4] and a typical TE material with low thermal conductivity [5-7]. Bi-Te thin films and nanostructures are important Bi2Te3 crystallizes into a layered structure with materials for thermoelectric (TE) and electronic quintuple layers ordered in a Te-Bi-Te-Bi-Te device applications. Stoichiometric Bi2Te3 (40% Bi sequence. Bi-Te films with other molar content and 60% Te) is of particular interest because it is a percentages such as BiTe (50% Bi and 50% Te) and representative topological insulator (TI) with a Bi4Te3 (57% Bi and 43% Te) have different Address correspondence to Rui He, [email protected] 2 Nano Res. crystalline structures [8, 9] and different vibrational contrast with the substrate. The difference in the properties [10] compared to the stoichiometric Raman peaks and color contrast in NPs with the Bi2Te3 counterpart. It has been shown that a same thickness is attributed to the variation in deviation (even as small as 0.1%) of Bi-Te atomic stoichiometry (confirmed by AES) and concomitant ratio from stoichiometry has a large impact on the change in the optical absorption coefficient α. An electronic and TE properties of Bi2Te3 material [11, α-versus-stoichiometry plot suggests that α may 12]. Moreover, fine tuning of the doping and experience a dramatic change when the Bi atomic stoichiometry of telluride and related content is about 45%. chalcogenides turns out to be central to the control of topological surface transport in the research of 2 Experimental procedures three-dimensional TIs [13-16]. Understanding the fundamental physical properties of Bi-Te thin films In this work Bi2Te3 NPs were grown by low and nanostructures with different thicknesses and pressure vapor transport with similar setup as in stoichiometries is thus crucial to various our previous work [17] and in the work done by applications of these materials. Kong et al [18]. Bi2Te3 flakes (99.999%, Alfa Aesar) In this paper we present our studies of Bi-Te were ground thoroughly. An amount of 80 mg of the resulting powder was transferred to a quartz nanoplates (NPs) grown by low pressure vapor crucible placed at the center of a single-zone tube transport method. We use Raman spectroscopy, furnace (Lindberg Blue M) as the vapor precursor atomic force microscopy (AFM), energy-dispersive for the growth. A 1 × 5 cm2 Si wafer with 300 nm X-ray spectroscopy (EDS), and Auger electron oxide layer was rinsed with acetone, ethanol and spectroscopy (AES) to characterize the composition, deionized (DI) water for 10 seconds each. Then the substrate wafer was dried with compressed morphology and to study the optical and air followed by ozone cleaning for 10 minutes at vibrational properties of Bi-Te thin layers. Raman room temperature. The cleaned wafer was placed scattering from relatively thick (at least tens of 11.5 cm away from the center of the furnace nanometers thick) Bi-Te NPs shows four Raman downstream. The system was pumped down to a peaks (at low laser power) that overlap those from base pressure of 20 mTorr and flushed with 100 standard cubic centimeters per minute (sccm) of bulk Bi2Te3, which indicates that these relatively Ar (10% H2) gas for 1 minute. The pressure was thick layers form Bi2Te3 bulk phase. At intermediate later adjusted to 500 mTorr, and the Ar flow rate laser power the surface of these Bi2Te3 layers forms was reduced to 40 sccm. To eliminate any residue a bump, and an increase of oxygen concentration is humidity in the system, the furnace was heated to observed at the laser spot in the EDS map. 100 oC for 30 minutes prior to the growth. The Characteristic Raman lines from Bi2O3 and TeO2 precursor (Bi2Te3 powder) temperature was then raised to 540 oC (leaving growth temperature on appear in the Raman spectrum. Therefore, Bi2Te3 the substrate to be approximately 420 oC) and surface is oxidized under laser heating with kept constant for 5 minutes. Once the growth intermediate power. As the laser power is further process finished, the system was cooled down to increased, holes are burned on the sample surface, the room temperature. and the depth of the holes increases at higher laser Photolithography process was conducted on power. another cleaned SiO2/Si wafer to make patterned metal features as references for samples’ In Bi-Te NPs thinner than 20 nm, Raman signals coordinates or bases for suspending samples. are different from those of Bi2Te3 phase. A new Using LOR3A and S1805 (MicrochemTM) mode centered at ~93 cm-1 appears in the Raman copolymer/polymer bilayer standard spin coating spectra, and the position of the highest frequency and baking recipe, photolithography patterns Raman mode varies significantly in NPs with even were made after UV light exposure over the photo mask for 2.50 seconds prior to the same thickness. Optical images show that NPs development in CD-26 (Microposit MF) with the same thickness have very different color developer for 70 seconds. The wafer was then | www.editorialmanager.com/nare/default.asp Nano Res. 3 rinsed in DI water for 30 seconds and dried with power increases to an intermediate level of 1 mW, compressed air. Afterwards, 5-nm-thick Cr and Raman spectrum from the same NP changes 70-nm-thick Ni were evaporated over the wafer dramatically. Raman intensities from all modes before the removal of the remaining photoresist increase significantly compared to those excited with PG remover solution. with low laser power. The Eg1, A1g1, and Eg2 modes Bi2Te3 NPs were transferred by contact printing all shift to lower frequency possibly due to method. The prepared SiO2/Si wafer with Ni expanded crystal lattice and reduced metal patterns was gently pressed against the interlayer/interatomic interactions at elevated growth wafer to transfer Bi-Te NPs and to create temperature. The most prominent change in the suspended NPs. The wafer with transferred NPs Raman spectrum is the emergence of two strong was later rinsed with Ethanol and DI water then and broad bands centered around 123 and 140 dried with compressed air for final cleaning prior cm-1 and a weak shoulder centered at about 91 to further characterization.