Electronic Supplementary Material

Structural and Electronic Investigation of Metal- Semiconductor Hybrid Tetrapod Heterostructures

Krishna Kanta Haldar1*, Vijaykumar Yogesh Muley2, Suwarna Datar3, and Amitava Patra4

1Centre for Chemical Sciences, School of Basic and Applied Sciences, Central University of Punjab, Bathinda, Punjab, 151001, India. 2Centre for Computational Sciences, School of Basic and Applied Sciences, Central University of Punjab, Bathinda, Punjab, 151001, India 3Department of Applied Physics, Defense Institute of Advanced Technology, Pune, 411025, India 4Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, 700032, India.

*Corresponding author: [email protected]

EXPERIMENTAL

Materials: Chloroauric acid (HAuCl4 ·3H2O, 99.9 %), Cadmium Oxide (CdO),Selenium powder (Se), stearic acid (SA, 95%), trioctylphosphine (TOP,90 % ), trioctylphosphine oxide (TOPO, 99 %), oleylamine (OAm, (98 %), hexadecyl amine (HDA, 90 %), 1-octadecene (99%), tetraoctylammonium bromide (TOAB, Fluka) and 1-dodecylamine (DDA, 98%) were purchased from Aldrich and sodium borohydride (NaBH4), toluene (chemical grade), n- hexane (analytical grade), methanol (analytical grade) and ethanol (analytical grade) were purchased from Merck India Ltd. All of the reagents were used without further purification.

Methods:

1 Synthesis of Au nanoparticles: Au nanoparticles were synthesized by methods modified from reported methods [1-4]. Typically, 20 ml of 5 mM HAuCl4 aqueous solution was mixed with 10 ml of 25 mM TOAB in toluene. After the Au precursor was transferred to the toluene layer, the toluene layer was mixed with 0.202 g dodecyl amine in 5ml toluene. Then, 0.054 g

NaBH4 in 3 ml H2O was dropwise added to the mixture with vigorous stirring. The mixture turned brown indicating the formation of Au nanoparticles. Au nanoparticles were precipitated from toluene by adding ethanol followed by centrifugation. The nanoparticles were re-dispersed in 10 ml hexane. For this particular procedure, the Au nanoparticles were about 4 nm in diameter, with the nearly spherical shape.

Synthesis of noble gold-cadmium selenide tetrapod nanocrystals: Stock solution of cadmium source has been prepared using CdO (50 mg) and stearic acid (SA, 350 mg) in 10 ml 1-octadecene (ODE). The mixture has been loaded in a 50 ml three-necked flask, degassed and heated to 250 °C to get a clear solution. Then the reaction was cooled to room temperature and solution were stored in a vial. A solution of TOP-Se was prepared by dissolving selenium (0.223 g) in a mixture of trioctylphosphine (TOP) (3 ml) and 1-octadecene (3 ml) at 50 °C temperature under stirring condition. In a separate reaction flask (50 ml) gold stock solution (see supporting information) in 10 ml toluene (OD = 0.34 at 520 nm) and 10 ml ODE are degassed and heated at 100oC to evaporate toluene. Then the reaction temperature is increased to 280 oC. 1 ml stock solution of Se has been injected at this temperature. Just after one minute, 1.5 ml cadmium stock solution has been swiftly injected. Different time intervals samples are taken out, washed with acetone and dispersed in chloroform.

Characterization: Absorption and fluorescence spectra of Au/CdSe tetrapod nanocrystals were taken at room temperature with a Shimadzu UV-2450 UV-VIS spectrometer. The crystalline phase of the nanoparticles was studied by X-ray diffraction patterns (Rich-Seifert XRD 3000P). High- resolution transmission electron microscope (HR-TEM) images (JEOL-TEM-2010 operating voltage at 200kV). High angle annular dark field scanning transmission electron microscopic image were done by Ultra High-Resolution field emission gun transmission electron microscope (JEOL 300kV). STM/STS measurements were performed at room temperature and ambient conditions with Nanosurf STM. Tetrapods were dispersed in toluene and then drop cast on highly oriented pyrolytic graphite (HOPG) substrate. Freshly cut Pt/Ir tip was used for imaging. The images were obtained at the bias voltage and current of 0.65V and 1nA respectively. For the I-V measurement the tip was retracted from the sample by increasing the tip-sample bias voltage to 1.25 V, thus attaining the double barrier tunnel junction (DBTJ) configuration such that maximum voltage was dropped across the tip - tetrapod junction[5-6].

2 This ensured that the valance band and conduction band states appear in dI/dV before the current reaches its maximum. dI/dV curves are characteristic of LDOS of the sample. LDOS at different positions of tetrapod gave information about the influence of Au particle at the interface of Au- CdSe on valance band and conduction band levels of CdSe.

a)

Figure S-1. HRTEM image of AuSe nanocrystals.

Figure S-2. HRTEM EDX analysis of AuSe nanocrystals.

3 Reference

1. Haldar, K. K.; Sinha, G.; Lahtinen, J. and Patra, A. Hybrid Colloidal Au-CdSe Pentapod Heterostructures Synthesis and Their Photocatalytic Properties. ACS Appl. Mater.Interfaces 2012, 4, 6266–6272. 2. Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of thiol- derivatised gold nanoparticles in a two-phase Liquid–Liquid system. J. Chem. Soc., Chem. Comm. 1994, 801-802. 3. Shi, W.; Zeng, H.; Sahoo, Y.; Ohulchanskyy Tymish, Y.; Ding, Y.; Wang Zhong, L.; Swihart, M.; Prasad P.N. A General Approach to Binary and Ternary Hybrid Nanocrystals. Nano Lett. 2006, 6, 875–881. 4. Haldar, K. K.; Pradhan, N.; and Patra, A. Formation of Heteroepitaxy in Different Shapes of Au–CdSe Metal–Semiconductor Hybrid Nanostructures. Small 2013, 9, 3424 5. Guisinger, N. P.; Greene, M. E.; Basu, R.; Baluch, A. S.; Hersam, M. C. Room Temperature Negative Differential Resistance through Individual Organic Molecules on Silicon Surfaces. Nano Lett. 2004, 4, 55-59. 6. Millo, O.; Katz, D.; Steiner, D.; Rothenberg, E.; Mokari, T.; Kazes, M.; Banin, U. Charging and Quantum Size Effects in Tunneling and Optical Spectroscopy of CdSe Nanorods. Nanotech.2004, 15, R1-R6.

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