High-Yield Gas-Phase Laser Photolysis Synthesis of Germanium

Nov 29, 2012 - Han Sung Kim, Seung Hyuk Back, Young Rok Lim, Chi-Woo Lee, and Jeunghee Park* ... Min Seob Song and Won-Il Cho. Center for Energy ...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCC

High-Yield Gas-Phase Laser Photolysis Synthesis of Germanium Nanocrystals for High-Performance Photodetectors and Lithium Ion Batteries Chang Hyun Kim, Hyung Soon Im, Yong Jae Cho, Chan Su Jung, Dong Myung Jang, Yoon Myung, Han Sung Kim, Seung Hyuk Back, Young Rok Lim, Chi-Woo Lee, and Jeunghee Park* Department of Chemistry, Korea University, Jochiwon 339-700, Korea

Min Seob Song and Won-Il Cho Center for Energy Convergence, Korea Institute of Science and Technology, Seoul 136-791, Korea S Supporting Information *

ABSTRACT: We developed a new high-yield synthesis method of free-standing germanium nanocrystals (Ge NCs) by means of the gas-phase photolysis of tetramethyl germanium in a closed reactor using a Nd:YAG pulsed laser. The as-grown Ge NCs are sheathed with thin (1−2 nm) carbon layers, and size control (5−100 nm) can be simply achieved using a quenching gas. The hybrid nanostructures of the quantum-confined Ge NCs and reduced graphene oxide (RGO) become transparent flexible photodetectors that respond sensitively over the UV−visible−NIR wavelength range. Both Ge NCs and Ge−RGO hybrids exhibit excellent cycling performance and high capacity of the lithium ion battery (800 and 1100 mA h/g after 50 cycles, respectively) as promising anode materials for the development of high-performance lithium batteries.

1. INTRODUCTION As one of the important IV semiconductor materials, germanium (Ge) has been the object of particular attention due to its narrow band gap (0.67 eV in the bulk form at room temperature) and its large Bohr radius (ca. rB = 24 nm), which provide a strong quantum confinement effect. 1−6 Ge nanostructures exhibit a wide range of potential applications in field effect transistors, solar cells, lithium ion batteries, photodetectors, IR light-emitting diodes, and biological photothermal therapy.7−19 To expand their practical applications, the synthesis of stable colloidal nanocrystals (NCs) is a desirable route toward cost-effective solution-based fabrication of quantum-confined thin films.20−22 Unfortunately, most solution-phase synthetic routes involve a time-consuming surface treatment process because of the ligands that usually hinder charge carrier transport between NCs after casting and thus render as-deposited films electrically insulating. As an approach to produce ligand-free Ge NCs, Kortshagen and co-workers recently developed a continuous-flow plasma process which dissociates GeCl4 vapor and H2 to produce the NC film.23 Erogbogbo et al. reported the gas-phase synthesis of freestanding Ge NCs using CO2 laser-induced pyrolysis of GeH4.24 We report a new strategy for the synthesis of ligand-free Ge NCs: a gas-phase photolysis of tetramethyl germanium (Ge(CH3)4, TMG) in a closed reaction vessel, using a 532 or 1064 nm Nd:YAG pulsed laser. This method has an advantage of high yield with excellent reproducibility compared to the previous gas flow system. Furthermore, the addition of © 2012 American Chemical Society

quenching gas simply allows the size control of the Ge NCs. We synthesized the stable hybrid form of Ge NCs with twodimensional carbon nanostructure reduced graphene oxide (RGO) without any covalent linker. The Ge NC hybrid nanostructures were considered as promising transparent flexible optoelectronic and high-capacity lithium ion battery anode materials. This high-yield, cost-effective synthesis technique is an important milestone on the way toward broad commercial applications of solar cells and lithium ion batteries.

2. EXPERIMENTAL SECTION Laser photolysis was performed using a Nd:YAG pulsed laser (Coherent SL-10) operating at 1064 nm or second-harmonic generated 532 nm with a repetition rate of 10 Hz and a pulse width of 10 ns. TMG (98%, Sigma-Aldrich) was degassed by several freeze (77 K)−pump−thaw cycles and then used without further purification. The precursor vapors (5−50 Torr) with or without the quenching gas (CO2, O2) were introduced to a 1 L volume pyrex glass reactor, equipped with a gas valve connecting to a standard vacuum line and a 2 in. diameter quartz optical window. The laser beam was focused into the reactor with a 10 cm focal length lens through the window. The experiment was carried out using a laser pulse energy of 0.1− Received: September 6, 2012 Revised: November 7, 2012 Published: November 29, 2012 26190

dx.doi.org/10.1021/jp308852g | J. Phys. Chem. C 2012, 116, 26190−26196

The Journal of Physical Chemistry C

Article

at 365 nm (max. 80 mW) and 850 nm (max. 170 mW) and a 514 nm Ar ion laser (10 mW) were used as the light source. For the electrochemical tests, the electrodes of the battery test cells were made of the active material (Ge and Ge−RGO), acetylene carbon black, and poly(acrylic acid) (PAA, 30 wt % dissolved in water; Aldrich) binder at a weight ratio of 5:3:2. The distilled water−mixed slurry was coated onto the 20 μm thick Cu foil. The coated electrode was dried at 80 °C for 12 h and then roll-pressed. The coin-type half cells (CR2032) prepared in a helium-filled glovebox contained an electrode, a Li metal anode, a microporous polyethylene separator, and an electrolyte solution of 1 M LiPF6 in 1:1:1 vol % ethylene carbonate:ethyl methyl carbonate:dimethyl carbonate.

0.2 J/pulse. After 1−3 h of laser irradiation, the gas products were vented and the free-standing NCs were collected by dispersing them in ethanol, followed by evaporation and vacuum drying at room temperature. Graphene oxide was synthesized from graphite by Hummers’ exfoliation method, and was reduced into RGO using heating in H2 flow at 800 °C for 1 h.25 The Ge NCs were dispersed with RGO in isopropyl alcohol (IPA) through 30 min of sonication and then precipitated and dried for TEM or other analyses. The products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700), field-emission transmission electron microscopy (TEM, Jeol JEM 2100F and FEI TECNAI G2 200 kV), high-voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV), and energy-dispersive X-ray fluorescence spectroscopy (EDX). High-resolution X-ray diffraction (XRD) patterns were obtained using the 9B and 3D beamlines of the Pohang Light Source (PLS) with monochromatic radiation (λ = 1.54595 Å). X-ray photoelectron spectroscopy (XPS) was performed using the 8A1 beamline of the PLS and a laboratorybased spectrometer (ESCALAB 250, VG Scientifics) using a photon energy of 1486.6 eV (Al Kα). Thermogravimetric analysis (TGA) was performed using a TA Instruments Ltd. SDT Q600 system. Samples were heated in a flow of N2/air mixture (100 sccm) at 10 °C/min from room temperature to 1000 °C. Elemental analysis was performed using Thermo Scientific Flash 2000. A UV−visible−NIR absorption spectrometer (Varian, Cary 1000) was used to measure the optical properties. The cyclic voltammetry (CV) curves were recorded on an IVIUM CompactStat electrochemical analyzer using gold (area ca. 0.02 cm2) disks as the working electrode, a Pt-wire auxiliary electrode, and a Ag/0.01 M AgNO3 reference electrode (BAS Inc., 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)/acetonitrile electrolyte). A drop of the 1 mg Ge NC or Ge powder (bulk) dispersed in 1 mL of IPA solution was placed on the polished surface of the working electrode and then the solvent was evaporated to form a film. The data of Ge powder was obtained after deleting the oxide layer on the surface by 5% HF aqueous solution. The electrolytes were thoroughly deoxygenated by bubbling with high-purity argon for 15 min. The VB and CB band energies were calculated from these EOx and ERed values, assuming the energy level of ferrocene/ferrocenium (Fc/Fc+) to be −4.8 eV below the vacuum level. The formal potential of Fc/Fc+ was measured to be 0.085 V against a Ag/Ag+ reference electrode. Therefore, EVB (EHOMO) = −(EOx + 4.715) eV; ECB (ELUMO) = −(ERed + 4.715) eV, where the onset potential values are relative to the Ag/Ag+ reference electrode. Photolithography was used to deposit the Ti (20 nm)/Au (80 nm) film electrode structure onto a Si substrate with a 1 μm thick thermally grown SiO2 layer, by sputtering using a patterned mask. The gap distance between the electrodes was 2 μm. The assembled electrodes were further fabricated by the dielectrophoresis (DEP) technique. A drop (0.2 μL) of the NC−RGO hybrids dispersed in IPA was then dropped between the electrodes on the substrate. An alternating electric field of 10 kHz with a peak-to-peak voltage V = 5 V was applied between the electrodes for 1 min. AFM (Park System XE100) confirmed that the NC−RGO sheets between the electrodes had an average thickness of 4 nm. These devices were heatannealed at 300 °C for 1 min. We tested the devices on the probe station with parametric test equipment (Agilent E5270A) at room temperature. A light emitting diode (LED) operating

3. RESULTS AND DISCUSSION Figure 1 is a sketch of the production of Ge NCs in the closed reaction vessel via photolysis of TMG vapors using a pulsed

Figure 1. Schematic diagram of a closed gas reactor undergoing the laser photolysis of TMG by a focused laser beam and a photograph visualizing high-yield production of Ge NCs inside the glass reactor after 1 h of laser photolysis.

laser. After 1−3 h of laser irradiation, the free-standing NCs were collected from the reactor. The photograph shows asgrown Ge NCs in the flask shaped pyrex glass reactor after 1 h of laser irradiation. This simple and cost-effective method allows a high yield such as 80% in 1 h; e.g., 20 Torr of TMG (1.1 mmol) filled in a 1 L reactor produces 70 mg (0.97 mmol) using 0.2 J/pulse at 1064 nm. A higher power pulse (if available) can produce the larger scaled NCs in a shorter time. The XRD patterns confirmed a highly pure cubic phase Ge, as shown in the Supporting Information, Figure S1. Figure 2a shows the TEM image of Ge NCs revealing a homogeneous size distribution with an average value of 9 ± 1 nm. Figure 2b shows the lattice-resolved and corresponding fast-Fourier-transform (FFT) images at the zone axis of facecentered cubic [011]. The (111) fringes are separated by a distance of about 3.3 Å, which is close to that of the cubic phase bulk Ge (JCPDS No. 04-0545; a = 5.6576 Å). The Ge NCs are usually sheathed with 1−2 nm thick C layers. It is clearly observed that 3−5 graphitic layers, whose adjacent layers are separated by a distance of 3.4 Å corresponding to the d-spacing of the graphite (002) planes, surround the Ge NCs (Figure 2c). We estimated the C content to be an average of 15 wt %, using EDX, TGA, and elemental analysis, as shown in the Supporting Information, Figure S2. The analysis of the fine-scanned XPS Ge 3d peak also supports the presence of surface C layers (Supporting Information, Figure S3). We observed that the size was not much changed over the pressure range 5−50 Torr of TMG. The addition of higher pressure nonreactive quenching gas such as CO2 induces size 26191

dx.doi.org/10.1021/jp308852g | J. Phys. Chem. C 2012, 116, 26190−26196

The Journal of Physical Chemistry C

Article

generated by the dissociation of TMG, they collide and aggregate into the Ge NCs. Ge atoms can also react with TMG and Ge-containing intermediates (Ge(CH3)3, etc.) and sequentially produce the Ge NCs. The carbons are produced during the dissociation and the growth reaction. The reactive Ge NCs undergo the quenching by colliding with the gas products and reactor wall, and then the growth reaction is terminated. The growth reaction can be briefly described by the following steps: (i) TMG + mhν (m > 1) → Ge* + 4CH3 (generation step of Ge atoms); (ii) Ge* + nTMG (or Ge-containing intermediates) → Gen* + carbon (growth step of Ge NCs); (iii) Gen* + Q → Gen + Q (quenching and termination step), where an asterisk (*) represents the “hot” reactive state; Gen and Q symbolize the Ge NCs and quenching gas, respectively. In the growth step, the C layers are deposited on the Ge NCs as a shell. The CO2-induced size reduction suggests that the collisional energy transfer from the reactive Ge NCs into the vibrational modes of CO2 decrease the rate of growth reaction. The addition of O2 may induce a higher growth rate via more reactive oxide radial intermediates (e.g., Ge + O2 → GeO + O; GeO + Ge* (or TMG) → Gen + O2), and thereby increase the size of NCs. The UV−visible spectrum of Ge powder (99.999%, Adrich) and Ge NC (diameter = 9 nm) colloid (in toluene) has been measured, as shown in Figure 3a. The spectrum was used to estimate the band (indirect) gaps by performing Kubelka− Munk transformations (Figure 3b).32 A plot of [αhν]1/2 (where α is the absorption coefficient) versus photon energy yielded a band gap of 0.6 and 1.0 eV, respectively, for the Ge powder and Ge NC. This result is consistent with the value of the bulk (0.67 eV) and the value of recent work on the Ge NCs; Eg (11 nm diameter) = 0.9 eV, respectively.33 The CV curve (vs Ag/Ag+ reference electrode) of Ge powder and Ge NC was measured using a scan rate of 20 mV s−1 (Figure 3c). It shows an oxidation peak at 0.01 and 0.02 V for the Ge powder and Ge NC, respectively. From this lowest oxidation peak (EOx), the position of the VB edge is estimated to be EVB = −4.73 and −4.74 eV, respectively. The value is close to the work function of the bulk (4.7 eV). The reduction peaks (ERed) appear at −0.70 and −0.98 V, respectively, for the Ge powder and Ge NC, suggesting ECB = −4.02 and −3.74 eV. The band gap of Ge powder and Ge NC is 0.7 and 1.0 eV, respectively, which is consistent with the value obtained from the UV−visible spectrum. The reduced band gap of Ge NCs is ascribed to their quantum-confinement effect. The Ge NCs are well dispersed in a variety of solvents such as methanol (MeOH), ethanol (EtOH), isopropyl alcohol (IPA), acetonitrile (AN), and benzonitrile (BN) (Figure 4a). The Ge NCs remain as a stable colloid solution, and also produce a homogeneous thin film (thickness = ∼2 μm) on the Si substrates using a spin coater. The solubility is comparable to that of ligand-free Ge NCs reported by other groups.23,24 The Ge NCs were dispersed in IPA and mixed thoroughly with RGO using sonication to synthesize the Ge−RGO hybrid nanostructures. Figure 4b shows the TEM image of the Ge−RGO hybrid nanostructures, synthesized using a 1:1 weight ratio of Ge:RGO. The Ge NCs were homogeneously attached onto the RGO sheets (thickness = 2−8 nm). The thin C layer of the Ge NCs would play the role as a proper linker for attachment to the RGO. We fabricated a number of photodetector devices by varying the wt % of the loaded NCs, and obtained the

Figure 2. (a) TEM image of the Ge NCs synthesized using the laser photolysis of TMG. The average diameter of 200 NCs is ∼9 nm. (b) Lattice-resolved and corresponding FFT images of selected NCs. The d-spacing of the Ge(111) planes is 3.3 Å. (c) The graphitic layers are separated by a distance of 3.4 Å, corresponding to the d-spacing of the graphite (002) planes. TEM images of (d) average 5 nm diameter Ge NCs synthesized using CO2 quenching gas and (e) average 20 nm and (f) average 100 nm diameter Ge NCs synthesized using O2 quenching gas.

reduction but amorphizes and decreases the yield. Figure 2d shows the TEM image for the average 5 nm diameter Ge NCs synthesized using a mixture of 20 Torr TMG and 200 Torr CO2. The addition of reactive quenching gas such as O2 increases the size of the NCs. Figure 2e and f corresponds to the TEM image for average 20 and 100 nm diameter Ge NCs produced in the presence of 20 and 100 Torr O2, respectively, during the photolysis. The XRD patterns confirmed highly pure cubic phase Ge without any oxide form (Supporting Information, Figure S1). Higher pressure O2 (>150 Torr) starts to produce the oxide form such as GeO2. In the past, ArF (193 nm) photolysis and the CO2 laserinduced pyrolysis of TMG (or tetraethyl germanium) in the gas phase, as one of the chemical vapor deposition methods, was used to produce Ge films.26−28 However, there have been no reports on the laser photolysis synthesis of free-standing Ge NCs. It is known that the irradiation of a focused laser beam induces the multiphoton absorption to dissociate into Ge atoms and the gas products consist of methane, ethane, ethene, etc.29,30 The Ge atoms can be either in the 4p2 ground state (3PJ) or electronically excited states (e.g., 1D2 (0.883 eV above), 1 S0 (2.029 eV above)).31 The two photons of 532 nm (2.32 eV) deliver sufficient energy to induce the cleavage of Ge−C (ca. 2.49 eV) and C−H (ca. 4.25 eV) bonds. Once the Ge atoms are 26192

dx.doi.org/10.1021/jp308852g | J. Phys. Chem. C 2012, 116, 26190−26196

The Journal of Physical Chemistry C

Article

Figure 3. (a) UV−visible absorption spectrum of Ge powder and Ge NC (diameter = 9 nm) colloid (in toluene) and (b) plot of [αhν]1/2 versus photon energy (eV). (c) CV curve (vs Ag/Ag+ reference electrode) of Ge powder and Ge NC.

Figure 4. (a) Photograph showing the Ge NCs (1 mg/mL) dispersed in MeOH, EtOH, IPA, AN, and BN. SEM image of the 2 μm thick Ge NC film deposited on the Si substrate, by spin coating of IPA colloid solution. (b) TEM image of the Ge−RGO and SEM image of the photodetector device fabricated using the DEP technique. A white dashed line denotes the RGO sheet as a guide. (c) I−V characteristics of photodetector under 365, 514, and 850 nm irradiation and in darkness. (d) I−t curves at a bias voltage of 2 V under chopped irradiation. A scheme for the photocurrent generation mechanism using a possible charge transfer model is shown.

RGO sheet is deposited between the 2 μm gap Ti/Au electrodes. Figure 4c shows the current−voltage (I−V) curves of the photodetector under dark conditions and irradiation of various

highest photocurrent at the 1:1 weight ratio. The inset corresponds to the SEM image showing the photodetector device fabricated using the DEP technique. The Ge NC-loaded 26193

dx.doi.org/10.1021/jp308852g | J. Phys. Chem. C 2012, 116, 26190−26196

The Journal of Physical Chemistry C

Article

Figure 5. Charge and discharge voltage profiles of coin-type half cells using (a) Ge NCs and (b) Ge−RGO hybrid for 1, 10, 30, and 50 cycles tested between 0.01 and 1.5 V, at a rate of 0.1C (160 mA/g). (c) Charge/discharge capacity vs cycle number for a half cell cycled with a rate of 0.1C. (d) Cycling performance as the rate increases from 0.1 to 5.0C.

mA h/g.11,19,34−41 Ge has been considered as the most promising candidate because of its high theoretical capacity of 1600 mA h/g owing to Li alloy formation (Li4.4Ge) which is also shown by another candidate material Si (4200 mA h/g).42 Although Ge has less capacity than that of Si, Ge has inherent advantages over Si in the respects of fast lithium-ion diffusivity (400 times faster than Si) and high electrical conductivity.43 Nevertheless, both candidate materials have had limited applications due to large volume changes during lithium insertion/desertion, leading to capacity fading and poor cycling life. To overcome this problem, Ge nanostructures were coated with C shells or hosted in the graphene.37−41 The C layers play a structural buffering role in minimizing the mechanical stress induced by the volume change. Graphene sheets afford good dispersion of the NCs and guarantee a high electrical conductivity of the overall electrode, which is good for the purpose of high rates.37,41 The present Ge NCs not only have naturally thin C layers but also form strongly binding RGO composites, as shown in the photocurrent measurement. Herein, we measured the charge/discharge capacity of lithium ion battery for the Ge NCs and Ge−RGO hybrid with a 9:1 Ge:RGO weight ratio. Figure 5a and b show their voltage profiles for 1, 10, 30, and 50 cycles at a charge/discharge rate of 0.1C (160 mA/g) tested between 0.01 and 1.5 V. The first discharge and charge capacities of the Ge NCs were 1600 (=1C) and 820 mA h/g, respectively, with an initial Coulombic efficiency of 50%. The 10 wt % RGO gives rise to a significant increase of the first charge/discharge capacity (2300 and 1200 mA h/g , respectively), with the initial Coulombic efficiency of 50%. This large initial capacity loss can be attributed to the formation of the solid electrolyte interphase (SEI) layers on the electrode surface during the first discharge step and the storage of Li cations. After the first cycle, both samples show nearly complete

light sources covering a wide range of wavelengths from UV to the near-infrared (NIR) region; 365 nm (80 mW, LED), 514.5 nm (10 mW, Ar ion laser), and 850 nm (170 mW, LED). The current of the “RGO only” devices showed no change upon illumination. The I−V characteristics are linear and symmetric within the measured range −2 to 2 V), indicating good ohmic contact of the devices. The current is drastically increased when irradiation is turned on. The photocurrent (ΔI) was max. 100 μA, and the photosensitivity, defined as the ratio of ΔI to that in the dark (I0), was ca. 250%. The current was measured in real time with a bias voltage of 2 V. As shown in Figure 4d, the I−t curves were collected with light on/off cycles. The current was instantly increased (99% for up to 50 cycles. Figure 5c shows the charge/discharge capacity as a function of the cycle number up to 50 cycles at a rate of 0.1C. Both samples exhibit very stable reversible capacities after the first cycle. The capacity of Ge−RGO remains at 1100 mA h/g after 50 cycles, which is larger than that (800 mA h/g) of the Ge NCs. The Coulombic efficiency of both samples remained at >99% after the initial cycle. Their rate capabilities were tested at the rates of 0.1, 0.2, 0.5, 1.0, 2.0, and 5.0C, and the results are plotted in Figure 5d. When the rate was increased to 1.0C, a small decrease in capacity was observed for both samples; from 1150 to 950 mA h/g for Ge−RGO; from 770 to 550 mA h/g for Ge NCs. Upon further increase of the rate to 5.0C, the capacity retention was 50% with a high Coulombic efficiency of 98%. When the rate was returned back to 0.1C after 60 cycles at the higher rates, the capacity of Ge NCs and Ge−RGO was increased back to 720 and 990 mA h/g, corresponding to 90%, of the initial capacity obtained at the 0.1C rate. Although the capacity of Ge NCs is lower than that of the Ge−RGO, the cycling performance is comparable to the Ge− RGO. This is probably due to the protective C layers that reduce pulverization during lithium insertion/desertion. The hybridization of highly conductive RGO leads to enhanced capacity in the first several cycles but also achieves a long cycle life and high power demands, as we expected. The capacity of Ge−RGO NCs is comparable to that (940 mAh/g) of Ge@C− RGO that was published recently.37 These results show that both Ge NCs and Ge−RGO hybrids are excellent anode materials for the development of high-performance lithium batteries.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was supported by NRF (2011-0015235; 20110020090; 2011-0013070; 2010-0029164), WCU (R31-2012000-10035-0), and KETEP(20104010100640; 2011201010016c; 20118510010030). The HVEM (Daejeon) and XPS (Pusan) were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH.

■

REFERENCES

(1) Heath, J. R.; Shiang, J. J.; Alivisatos, A. P. J. Chem. Phys. 1994, 101, 1607−1615. (2) Maeda, Y. Phys. Rev. B 1995, 51, 1658−1670. (3) Bostedt, C.; Buuren, T. V.; Willey, T. M.; Franco, N.; Terminello, L. J.; Heske, C.; Möller, T. Appl. Phys. Lett. 2004, 84, 4056−4058. (4) Lee, D. C.; Pietryga, J. M.; Robel, I.; Werder, D. J.; Schaller, R. D.; Klimov, V. I. J. Am. Chem. Soc. 2009, 131, 3436−3437. (5) Ruddy, D. A.; Johnson, J. C.; Smith, E. R.; Neale, N. R. ACS Nano 2010, 4, 7459−7466. (6) Armatas, G. S.; Kanatzidis, M. G. Nano Lett. 2010, 10, 3330− 3336. (7) Wang, D.; Wang, Q.; Javey, A.; Tu, R.; Dai, H.; Kim, H.; McIntyre, P. C.; Krishnamohan, T.; Saraswat, K. C. Appl. Phys. Lett. 2003, 83, 2432−2434. (8) Greytak, A. B.; Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. Appl. Phys. Lett. 2004, 84, 4176−4178. (9) Hanrath, T.; Korgel, B. A. J. Am. Chem. Soc. 2004, 126, 15466− 15472. (10) Lambert, T. N.; Andrews, N. L.; Gerung, H.; Boyle, T. J.; Oliver, J. M.; Wilson, B. S.; Han, S. M. Small 2007, 3, 691−699. (11) Chan, C. K.; Zhang, X. F.; Cui, Y. Nano Lett. 2008, 8, 307−309. (12) Meng, X.; Al-Salman, R.; Zhao, J.; Borissenko, N.; Li, Y.; Endres, F. Angew. Chem., Int. Ed. 2009, 48, 2703−2707. (13) Sun, B.; Zou, G.; Shen, X.; Zhang, X. Appl. Phys. Lett. 2009, 94 (233504), 1−3. (14) Assefa, S.; Xia, F.; Vlasov, Y. A. Nature 2010, 464, 80−85. (15) Michel, J.; Liu, J.; Kimerling, L. C. Nat. Photonics 2010, 4, 527− 534. (16) Henderson, E. J.; Seino, M.; Puzzo, D. P.; Ozin, G. A. ACS Nano 2010, 4, 7683−7691. (17) Cao, L.; Park, J.-S.; Fan, P.; Clemens, B.; Brongersma, M. L. Nano Lett. 2010, 10, 1229−1233. (18) Xue, D.-J.; Wang, J.-J.; Wang, Y.-Q.; Xin, S.; Guo, Y.-G.; Wan, L.-J. Adv. Mater. 2011, 23, 3704−3707. (19) Park, M.-H.; Cho, Y.; Kim, K.; Kim, J.; Liu, M.; Cho, J. Angew. Chem., Int. Ed. 2011, 50, 9647−6950. (20) Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2008, 8, 3488−3492. (21) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Science 2011, 334, 1530−1533. (22) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; et al. Nat. Mater. 2011, 10, 765−771. (23) Holman, Z. C.; Kortshagen, U. R. Nano Lett. 2011, 11, 2133− 2136. (24) Erogbogbo, F.; Liu, T.; Ramadurai, N.; Tuccarione, P.; Lai, L.; Swihart, M. T.; Prasad, P. N. ACS Nano 2011, 5, 7950−7959. (25) Hummers, W. S., Jr.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (26) Pola, J.; Taylor, R. J. Organomet. Chem. 1992, 437, 271−278.

4. CONCLUSIONS The Ge NCs (average diameter = 9 nm) were synthesized by the gas-phase photolysis of TMG using a 532 or 1064 nm Nd:YAG pulsed laser in a closed reactor. The yield of 1 h of photolysis reaction reached 80%, which indicates a possible gram scaled mass production. The size was reduced to 5 nm by adding nonreactive CO2 or increased up to 100 nm using reactive quenching with O2. These ligand-free NCs are naturally sheathed with thin C layers (1−2 nm thickness). The as-grown Ge NCs form a stable colloidal solution; thus, they are mixed with RGO to synthesize hybrid nanostructures. The Ge−RGO hybrid nanostructure produces a high photocurrent over a wide range of wavelengths (365, 514.5, and 850 nm) due to a strong quantum-confinement effect of the Ge NCs, which promises outstanding transparent flexible photodetectors. We measured stable cycling performance and high capacity of the lithium ion battery, 800 mA h/g after 50 cycles, for the Ge NCs. The Ge−RGO hybrid exhibited a greatly improved capacity, 1100 mA h/g after 50 cycles. This new strategy for the synthesis of high-quality Ge NCs guarantees promising applications in optoelectronic devices and lithium batteries. It is also expected to contribute to expand their applications in high-performance energy conversion systems.

■

Article

ASSOCIATED CONTENT

* Supporting Information S

EDX, XRD, and XPS data. This material is available free of charge via the Internet at http://pubs.acs.org. 26195

dx.doi.org/10.1021/jp308852g | J. Phys. Chem. C 2012, 116, 26190−26196

The Journal of Physical Chemistry C

Article

(27) Jakoubkova, M.; Bastl, Z.; Fiedler, P.; Pola, J. Infrared Phys. Technol. 1994, 35, 633−635. (28) Ishihara, F.; Uji, H.; Kamimura, T. Jpn. J. Appl. Phys. 1995, 34, 2229−2234. (29) Stanley, A. E. J. Photochem. Photobiol., A 1996, 99, 1−7. (30) Antman, M.; Hilt, L.; Christophy, E.; Belbruno, J. J. Chem. Phys. Lett. 1994, 221, 294−300. (31) Husain, D.; Ioannou, A. X.; Kabir, M. J. Photochem. Photobiol., A 1997, 110, 213−220. (32) Hagfeldtt, A.; Grätzel, M. Chem. Rev. 1995, 95, 49−68. (33) Holman, Z. C.; Kortshagen, U. R. Appl. Phys. Lett. 2012, 100 (133108), 1−4. (34) Wang, X.-L.; Han, W.-Q.; Chen, H.; Bai, J.; Tyson, T. A.; Yu, X.Q.; Wang, X.-J.; Yang, X.-Q. J. Am. Chem. Soc. 2011, 133, 20692− 20695. (35) Song, T.; Cheng, H.; Choi, H.; Lee, J.-H.; Han, H.; Lee, D. H.; Yoo, D. S.; Kwon, M.-S.; Choi, J.-M.; Doo, S. G.; et al. ACS Nano 2012, 6, 303−309. (36) Rudawski, N. G.; Darby, B. L.; Yates, B. R.; Jones, K. S.; Elliman, R. G.; Volinsky, A. A. Appl. Phys. Lett. 2012, 100, 083111. (37) Xue, D.-J.; Xin, S.; Yan, Y.; Jiang, K.-C.; Yin, Y.-X.; Guo, Y.-G.; Wan, L.-J. J. Am. Chem. Soc. 2012, 134, 2512−2515. (38) Tan, L. P.; Lu, Z.; Tan, H. T.; Zhu, J.; Rui, X.; Yan, Q.; Hng, H. H. J. Power Sources 2012, 206, 253−258. (39) Jo, G.; Choi, I.; Ahn, H.; Park, M. J. Chem. Commun. 2012, 48, 3987−3989. (40) Seng, K. H.; Park, M. H.; Cuo, Z. P.; Liu, H. K.; Cho, J. Angew. Chem., Int. Ed. 2012, 51, 5657−5661. (41) Chockla, A. M.; Panthani, M. G.; Holmberg, V. C.; Hessel, C. M.; Reid, D. K.; Bogart, T. D.; Harris, J. T.; Mullins, C. B.; Korgel, B. A. J. Phys. Chem. C 2012, 116, 11917−11923. (42) John, M. R., St.; Furgala, A. J.; Sammells, A. F. J. Electrochem. Soc. 1982, 129, 246−250. (43) Graetz, J.; Ahn, C. C.; Yazami, R.; Fultz, B. J. Electrochem. Soc. 2004, 151, A698−A702.

26196

dx.doi.org/10.1021/jp308852g | J. Phys. Chem. C 2012, 116, 26190−26196