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COLLOIDAL PREPARATION OF MONODISPERSE NANOCRYSTALS

Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 3 Research Link, Singapore 117602, Singapore

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MICHELLE D. REGULACIO

Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 3 Research Link, Singapore 117602, Singapore

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KWOK WEI SHAH

Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 3 Research Link, Singapore 117602, Singapore

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THAMMANOON SREETHAWONG

Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 3 Research Link, Singapore 117602, Singapore

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YUANGANG ZHENG

Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 3 Research Link, Singapore 117602, Singapore

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, and

MING-YONG HAN

Institute of Materials Research and Engineering, Agency for Science, Technology and Research, 3 Research Link, Singapore 117602, Singapore

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https://doi.org/10.1142/S2251237314300010Cited by:6 (Source: Crossref)

Abstract

In the field of nanoscience, the ability to prepare high-quality nanocrystals is crucial for fundamental research and technology development. Herein, we review a general route for the colloidal preparation of monodisperse nanocrystals, which is divided into five different stages, namely (i) nucleation, (ii) growth, (iii) Ostwald ripening, (iv) surfactant capping and (v) precipitation. Each of the five stages is discussed in detail to provide a comprehensive understanding of the mechanism behind nanocrystal formation and the subsequent processing steps. We put emphasis on the classical theories and experimental techniques that are most frequently practiced. These are useful for the successful design and fabrication of nanocrystals with properties that are beneficial for use in a plethora of technological applications.

Keywords:

References

B. D. Fahlman, Materials Chemistry (Springer, Mount Pleasant, Michigan, 2007) pp. 282–283. Crossref, Google Scholar
C. B. Murray , C. R. Kagan and M. G. Bawendi , Annu. Rev. Mater. Sci. 30 , 545 ( 2000 ) . Crossref, Google Scholar
C. N. R. Rao and A. K. Cheetham , J. Mater. Chem. 11 , 2887 ( 2001 ) . Crossref, Google Scholar
P. Yang, Encyclopedia of Physical Science and Technology, 3rd edn., ed. A. M. Robert (Academic Press, New York, 2003) pp. 319–334. Crossref, Google Scholar
Y. Xia et al. , Angew. Chem. Int. Ed. 48 , 60 ( 2009 ) . Crossref, Google Scholar
H. Li , A. G. Kanaras and L. Manna , Acc. Chem. Res. 46 , 1387 ( 2013 ) . Crossref, Google Scholar
V. K. Varadanet al., Nanoscience and Nanotechnology in Engineering (World Scientific Publishing, Singapore, 2010) pp. 176–179. Link, Google Scholar
J. M. Luther et al. , Nano Lett. 8 , 3488 ( 2008 ) . Crossref, Google Scholar
Z. Tan et al. , Adv. Mater. 23 , 3553 ( 2011 ) . Crossref, Google Scholar
M. Han et al. , Nat. Biotech. 19 , 631 ( 2001 ) . Crossref, Google Scholar
Z. Wang et al. , Anal. Chem. 77 , 5770 ( 2005 ) . Crossref, Google Scholar
M. M.-C. Cheng et al. , Curr. Opin. Chem. Biol. 10 , 11 ( 2006 ) . Crossref, Google Scholar
M. Ferrari , Nat. Rev. Cancer 5 , 161 ( 2005 ) . Crossref, Google Scholar
M. Ferrari , Nat. Nanotechnol. 3 , 131 ( 2008 ) . Crossref, Google Scholar
K. Kneipp et al. , Chem. Rev. 99 , 2957 ( 1999 ) . Crossref, Google Scholar
Y. Cui and C. M. Lieber , Science 291 , 851 ( 2001 ) . Crossref, Google Scholar
B. Amir Parviz , D. Ryan and G. M. Whitesides , IEEE Trans. Adv. Pack. 26 , 233 ( 2003 ) . Crossref, Google Scholar
D. Whang et al. , Nano Lett. 3 , 1255 ( 2003 ) . Crossref, Google Scholar
D. V. Talapin et al. , Chem. Rev. 110 , 389 ( 2010 ) . Crossref, Google Scholar
C. Ye et al. , Chem. Eur. J. 18 , 11258 ( 2012 ) . Crossref, Google Scholar
M. D. Regulacio et al. , CrystEngComm 15 , 5214 ( 2013 ) . Crossref, Google Scholar
X. J. Loh , W. Guerin and S. M. Guillaume , J. Mater. Chem. 22 , 21249 ( 2012 ) . Crossref, Google Scholar
C. W. T. Leung , Y. Hong and B. Z. Tang , J. Mol. Eng. Mater. 1 , 1340005 ( 2013 ) . Link, Google Scholar
E. Ye , G. R. Deen and X. J. Loh , J. Mol. Eng. Mater. 1 , 1330002 ( 2013 ) . Link, Google Scholar
K. Zhou and Y. Li , Angew. Chem. Int. Ed. 51 , 602 ( 2012 ) . Crossref, Google Scholar
Z.-Y. Zhou et al. , Chem. Soc. Rev. 40 , 4167 ( 2011 ) . Crossref, Google Scholar
H. Zhang et al. , Acc. Chem. Res. 46 , 1783 ( 2013 ) . Crossref, Google Scholar
Z. Zhuang , Q. Peng and Y. Li , Chem. Soc. Rev. 40 , 5492 ( 2011 ) . Crossref, Google Scholar
A. M. Smith and S. Nie , Acc. Chem. Res. 43 , 190 ( 2009 ) . Crossref, Google Scholar
M. D. Regulacio et al. , Chem. Mater. 20 , 3368 ( 2008 ) . Crossref, Google Scholar
V. F. Puntes , K. M. Krishnan and A. P. Alivisatos , Science 291 , 2115 ( 2001 ) . Crossref, Google Scholar
J. J. Li et al. , J. Am. Chem. Soc. 125 , 12567 ( 2003 ) . Crossref, Google Scholar
M. Law , J. Goldberger and P. Yang , Annu. Rev. Mater. Res. 34 , 83 ( 2004 ) . Crossref, Google Scholar
X. Wang et al. , Nature 437 , 121 ( 2005 ) . Crossref, Google Scholar
G. Wang , Q. Peng and Y. Li , Acc. Chem. Res. 44 , 322 ( 2011 ) . Crossref, Google Scholar
Y. Yin and A. P. Alivisatos , Nature 437 , 664 ( 2005 ) . Crossref, Google Scholar
P. D. Cozzoli , T. Pellegrino and L. Manna , Chem. Soc. Rev. 35 , 1195 ( 2006 ) . Crossref, Google Scholar
R. Costi , A. E. Saunders and U. Banin , Angew. Chem. Int. Ed. 49 , 4878 ( 2010 ) . Crossref, Google Scholar
M. D. Regulacio et al. , J. Am. Chem. Soc. 133 , 2052 ( 2011 ) . Crossref, Google Scholar
S. Behrens , Nanoscale 3 , 877 ( 2011 ) . Crossref, Google Scholar
M. D. Regulacio and M.-Y. Han , Acc. Chem. Res. 43 , 621 ( 2010 ) . Crossref, Google Scholar
N. S. Porter et al. , Acc. Chem. Res. 46 , 1867 ( 2013 ) . Crossref, Google Scholar
W. P. Lim et al. , Chem. Mater. 18 , 6170 ( 2006 ) . Crossref, Google Scholar
S. K. Batabyal et al. , J. Phys. Chem. C 113 , 15037 ( 2009 ) . Crossref, Google Scholar
M. D. Regulacio et al. , Chem. Eur. J. 18 , 3127 ( 2012 ) . Crossref, Google Scholar
M. Faraday , Phil. Trans. R. Soc. Lond. 147 , 145 ( 1857 ) . Crossref, Google Scholar
C. B. Murray et al. , IBM J. Res. Dev. 45 , 47 ( 2001 ) . Crossref, Google Scholar
C. Sun and D. Xue , Curr. Opin. Chem. Eng. 1 , 108 ( 2012 ) . Crossref, Google Scholar
Y. Kousaka , T. Nomura and M. Alonso , Adv. Powder Technol. 12 , 291 ( 2001 ) . Crossref, Google Scholar
B. B. Schaack et al. , Chem. Mater. 21 , 4448 ( 2009 ) . Crossref, Google Scholar
C. Desgranges and J. Delhommelle , J. Phys. Chem. C 113 , 3607 ( 2009 ) . Crossref, Google Scholar
P. M. Allen , B. J. Walker and M. G. Bawendi , Angew. Chem. Int. Ed. 49 , 760 ( 2010 ) . Crossref, Google Scholar
T. Yao et al. , J. Am. Chem. Soc. 132 , 7696 ( 2010 ) . Crossref, Google Scholar
K. Chaudhari , P. L. Xavier and T. Pradeep , ACS Nano 5 , 8816 ( 2011 ) . Crossref, Google Scholar
J. Song , D. Kim and D. Lee , Langmuir 27 , 13854 ( 2011 ) . Crossref, Google Scholar
R. Mohan and A. S. Myerson , Chem. Eng. Sci. 57 , 4277 ( 2002 ) . Crossref, Google Scholar
V. K. LaMer and R. H. Dinegar , J. Am. Chem. Soc. 72 , 4847 ( 1950 ) . Crossref, Google Scholar
C. B. Murray , D. J. Norris and M. G. Bawendi , J. Am. Chem. Soc. 115 , 8706 ( 1993 ) . Crossref, Google Scholar
C. R. Bullen and P. Mulvaney , Nano Lett. 4 , 2303 ( 2004 ) . Crossref, Google Scholar
E. Ye et al. , Chem. Eur. J. 17 , 5982 ( 2011 ) . Crossref, Google Scholar
J. Zhang and J. Fang , J. Am. Chem. Soc. 131 , 18543 ( 2009 ) . Crossref, Google Scholar
N. Pradhan , B. Katz and S. Efrima , J. Phys. Chem. B 107 , 13843 ( 2003 ) . Crossref, Google Scholar
Z. Zhuang et al. , Chem. Eur. J. 17 , 10445 ( 2011 ) . Crossref, Google Scholar
J. Park et al. , Nat. Mater. 3 , 891 ( 2004 ) . Crossref, Google Scholar
E. Ye et al. , Phys. Chem. Chem. Phys. 12 , 11923 ( 2010 ) . Crossref, Google Scholar
N. R. Jana , L. Gearheart and C. J. Murphy , Langmuir 17 , 6782 ( 2001 ) . Crossref, Google Scholar
B. Nikoobakht and M. A. El-Sayed , Chem. Mater. 15 , 1957 ( 2003 ) . Crossref, Google Scholar
S. H. Yu , H. Cölfen and M. Antonietti , Adv. Mater. 15 , 133 ( 2003 ) . Crossref, Google Scholar
S. E. Habas et al. , Nat. Mater. 6 , 692 ( 2007 ) . Crossref, Google Scholar
M. R. Langille et al. , J. Am. Chem. Soc. 134 , 14542 ( 2012 ) . Crossref, Google Scholar
T. K. Sau and C. J. Murphy , J. Am. Chem. Soc. 126 , 8648 ( 2004 ) . Crossref, Google Scholar
X. Li et al. , J. Am. Chem. Soc. 133 , 672 ( 2010 ) . Crossref, Google Scholar
J. Lian et al. , J. Am. Chem. Soc. 134 , 8754 ( 2012 ) . Crossref, Google Scholar
Y. Liao et al. , Adv. Mater. 24 , OP159 ( 2012 ) . Google Scholar
L. Carbone and P. D. Cozzoli , Nano Today 5 , 449 ( 2010 ) . Crossref, Google Scholar
Z. W. Seh et al. , Angew. Chem. Int. Ed. 50 , 10140 ( 2011 ) . Crossref, Google Scholar
C. D. M. Donega , Chem. Soc. Rev. 40 , 1512 ( 2011 ) . Crossref, Google Scholar
B. O. Dabbousi et al. , J. Phys. Chem. B 101 , 9463 ( 1997 ) . Crossref, Google Scholar
X. Zhong et al. , J. Am. Chem. Soc. 125 , 8589 ( 2003 ) . Crossref, Google Scholar
M. D. Regulacio et al. , Nanoscale 5 , 2322 ( 2013 ) . Crossref, Google Scholar
A. E. Saunders , I. Popov and U. Banin , J. Phys. Chem. B 110 , 25421 ( 2006 ) . Crossref, Google Scholar
J. Y. Rempel , M. G. Bawendi and K. F. Jensen , J. Am. Chem. Soc. 131 , 4479 ( 2009 ) . Crossref, Google Scholar
D. W. Oxtoby , Acc. Chem. Res. 31 , 91 ( 1998 ) . Crossref, Google Scholar
D. Erdemir , A. Y. Lee and A. S. Myerson , Acc. Chem. Res. 42 , 621 ( 2009 ) . Crossref, Google Scholar
J. Baumgartner et al. , Nat. Mater. 12 , 310 ( 2013 ) . Crossref, Google Scholar
J. Turkevich , P. C. Stevenson and J. Hillier , Discuss. Faraday Soc. 11 , 55 ( 1951 ) . Crossref, Google Scholar
R. Viswanatha and D. D. Sarma, Nanomaterials Chemistry: Recent Developments and New Directions, eds. C. N. R. Rao, A. Müller and A. K. Cheetham (WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2007) pp. 141–151. Google Scholar
C. D. Dushkin et al. , Adv. Colloid Interface Sci. 88 , 37 ( 2000 ) . Crossref, Google Scholar
E. E. Finney and R. G. Finke , J. Colloid Interface Sci. 317 , 351 ( 2008 ) . Crossref, Google Scholar
Z. H. Sun et al. , J. Phys. Chem. C 113 , 18608 ( 2009 ) . Crossref, Google Scholar
H. Oyanagi et al. , J. Synchrotron Radiat. 18 , 272 ( 2011 ) . Crossref, Google Scholar
S. Cheong et al. , J. Am. Chem. Soc. 131 , 14590 ( 2009 ) . Crossref, Google Scholar
Y. Mikhlin et al. , J. Colloid Interface Sci. 362 , 330 ( 2011 ) . Crossref, Google Scholar
H. Wei et al. , Nat. Nanotechnol. 6 , 93 ( 2011 ) . Crossref, Google Scholar
Y.-C. Wang and S. Gunasekaran , J. Nanopart. Res. 14 , 1 ( 2012 ) . Google Scholar
K. Yu , J. Ouyang and D. M. Leek , Small 7 , 2250 ( 2011 ) . Crossref, Google Scholar
X. Peng , J. Wickham and A. P. Alivisatos , J. Am. Chem. Soc. 120 , 5343 ( 1998 ) . Crossref, Google Scholar
A. C. Dharmaratne , T. Krick and A. Dass , J. Am. Chem. Soc. 131 , 13604 ( 2009 ) . Crossref, Google Scholar
A. Helfrich and J. Bettmer , Int. J. Mass Spectrom. 307 , 92 ( 2011 ) . Crossref, Google Scholar
S.-Y. Zhang et al. , Adv. Mater. 24 , 4369 ( 2012 ) . Crossref, Google Scholar
W. W. Yu et al. , Chem. Mater. 15 , 2854 ( 2003 ) . Crossref, Google Scholar
M. Tiemann et al. , J. Phys. Chem. B 110 , 23142 ( 2006 ) . Crossref, Google Scholar
R. Xie , Z. Li and X. Peng , J. Am. Chem. Soc. 131 , 15457 ( 2009 ) . Crossref, Google Scholar
W. Ostwald , Lehrbuch der Allgemeinen Chemie 2 ( Leipzig, W. Engelmann , Germany , 1896 ) . Google Scholar
W. Ostwald , Z. Phys. Chem. 37 , 385 ( 1901 ) . Google Scholar
P. W. Voorhees , Annu. Rev. Mater. Sci. 22 , 197 ( 1992 ) . Crossref, Google Scholar
A. Baldan , J. Mater. Sci. 37 , 2171 ( 2002 ) . Crossref, Google Scholar
G. Madras and B. J. McCoy , Chem. Eng. Sci. 59 , 2753 ( 2004 ) . Crossref, Google Scholar
W. L. Noorduin et al. , Angew. Chem. Int. Ed. 48 , 9600 ( 2009 ) . Crossref, Google Scholar
J. A. Thompson et al. , Micropor. Mesopor. Mat. 158 , 292 ( 2012 ) . Crossref, Google Scholar
Y.-M. Sung et al. , J. Phys. Chem. C 111 , 1239 ( 2007 ) . Crossref, Google Scholar
W.-S. Wang et al. , J. Phys. Chem. C 112 , 19390 ( 2008 ) . Crossref, Google Scholar
H. Zhao , M. Chaker and D. Ma , J. Phys. Chem. C 113 , 6497 ( 2009 ) . Crossref, Google Scholar
P. Dagtepe and V. Chikan , J. Phys. Chem. C 114 , 16263 ( 2010 ) . Crossref, Google Scholar
T. Sugimoto and F. Shiba , J. Phys. Chem. B 103 , 3607 ( 1999 ) . Crossref, Google Scholar
V. B. Fenelonov , G. G. Kodenyov and V. G. Kostrovsky , J. Phys. Chem. B 105 , 1050 ( 2001 ) . Crossref, Google Scholar
I. M. Lifshitz and V. V. Slyozov , J. Phys. Chem. Solids 19 , 35 ( 1961 ) . Crossref, Google Scholar
C. Wagner , Z. Elektrochem. 65 , 581 ( 1961 ) . Google Scholar
M. E. Norako , M. A. Franzman and R. L. Brutchey , Chem. Mater. 21 , 4299 ( 2009 ) . Crossref, Google Scholar
G. Madras and B. J. McCoy , Chem. Eng. Sci. 57 , 3809 ( 2002 ) . Crossref, Google Scholar
M. Uwaha and K. Koyama , J. Cryst. Growth 312 , 1046 ( 2010 ) . Crossref, Google Scholar
A. L. Rogach et al. , Adv. Funct. Mater. 12 , 653 ( 2002 ) . Crossref, Google Scholar
H. G. Yang and H. C. Zeng , J. Phys. Chem. B 108 , 3492 ( 2004 ) . Crossref, Google Scholar
B. Liu and H. C. Zeng , Small 1 , 566 ( 2005 ) . Crossref, Google Scholar
J. Li and H. C. Zeng , J. Am. Chem. Soc. 129 , 15839 ( 2007 ) . Crossref, Google Scholar
W.-S. Wang et al. , ACS Appl. Mater. Interfaces 1 , 780 ( 2009 ) . Crossref, Google Scholar
S. Chen et al. , Cryst. Growth Des. 10 , 1257 ( 2010 ) . Crossref, Google Scholar
X. Tian et al. , Cryst. Growth Des. 10 , 3990 ( 2010 ) . Crossref, Google Scholar
L. Zhang and H. Wang , ACS Nano 5 , 3257 ( 2011 ) . Crossref, Google Scholar
H. Chun Zeng , Curr. Nanosci. 3 , 177 ( 2007 ) . Crossref, Google Scholar
L. Zhang and H. Wang , J. Phys. Chem. C 115 , 18479 ( 2011 ) . Crossref, Google Scholar
Y. Sun and Y. Xia , Science 298 , 2176 ( 2002 ) . Crossref, Google Scholar
M. K. Naskar , A. Patra and M. Chatterjee , J. Colloid Interface Sci. 297 , 271 ( 2006 ) . Crossref, Google Scholar
F. Wang and W. E. Buhro , J. Am. Chem. Soc. 134 , 5369 ( 2012 ) . Crossref, Google Scholar
Y. Wu , D. Wang and Y. Li , Chem. Soc. Rev. ( 2014 ) , DOI: 10.1039/C3CS60221D . Google Scholar
A. Chatterjee et al. , Colloids Surf. A 297 , 258 ( 2007 ) . Crossref, Google Scholar
G. S. Chaubey et al. , J. Am. Chem. Soc. 129 , 7214 ( 2007 ) . Crossref, Google Scholar
W. Bu et al. , J. Phys. Chem. C 113 , 12176 ( 2009 ) . Crossref, Google Scholar
S. Carenco et al. , Chem. Mater. 22 , 1340 ( 2010 ) . Crossref, Google Scholar
P. de la Presa et al. , J. Appl. Phys. 100 , 123915 ( 2006 ) . Crossref, Google Scholar
S. K. Mehta , S. Kumar and M. Gradzielski , J. Colloid Interface Sci. 360 , 497 ( 2011 ) . Crossref, Google Scholar
S. Carenco et al. , New J. Chem. 37 , 1231 ( 2013 ) . Crossref, Google Scholar
S. Mourdikoudis and L. M. Liz-Marzán , Chem. Mater. 25 , 1465 ( 2013 ) . Crossref, Google Scholar
C. Amiens et al. , New J. Chem. 37 , 3374 ( 2013 ) . Crossref, Google Scholar
J. M. Pettibone and N. R. Reardon , Nanoscale 4 , 5593 ( 2012 ) . Crossref, Google Scholar
P. D. Cozzoli , A. Kornowski and H. Weller , J. Am. Chem. Soc. 125 , 14539 ( 2003 ) . Crossref, Google Scholar
F. Dubois et al. , J. Am. Chem. Soc. 129 , 482 ( 2006 ) . Crossref, Google Scholar
C.-A. J. Lin et al. , Small 4 , 334 ( 2008 ) . Crossref, Google Scholar
K. Luo et al. , Langmuir 24 , 11497 ( 2008 ) . Crossref, Google Scholar
P. De la Presa et al. , IEEE Trans. Magn. 44 , 2816 ( 2008 ) . Crossref, Google Scholar
Y. Wang et al. , Adv. Powder Technol. 21 , 203 ( 2010 ) . Crossref, Google Scholar
D. V. Talapin et al. , Physica E 14 , 237 ( 2002 ) . Crossref, Google Scholar
S. Behrens et al. , J. Phys.: Condens. Matter 18 , S2543 ( 2006 ) . Crossref, Google Scholar
N. Matoussevitch et al. , J. Magn. Magn. Mater. 311 , 92 ( 2007 ) . Crossref, Google Scholar
E. Ye et al. , Chem. Eur. J. 17 , 3074 ( 2011 ) . Crossref, Google Scholar
J. A. Jamison et al. , ACS Nano 2 , 311 ( 2008 ) . Crossref, Google Scholar
G. Chen et al. , J. Am. Chem. Soc. 131 , 4218 ( 2009 ) . Crossref, Google Scholar
L. Bai et al. , J. Am. Chem. Soc. 132 , 2333 ( 2010 ) . Crossref, Google Scholar
O. Akbulut et al. , Nano Lett. 12 , 4060 ( 2012 ) . Crossref, Google Scholar
T. Vossmeyer et al. , J. Phys. Chem. 98 , 7665 ( 1994 ) . Crossref, Google Scholar
D. V. Talapin et al. , Colloids Surf. A 202 , 145 ( 2002 ) . Crossref, Google Scholar
A. Akthakul et al. , Adv. Mater. 17 , 532 ( 2005 ) . Crossref, Google Scholar
B. Kowalczyk , I. Lagzi and B. A. Grzybowski , Curr. Opin. Colloid Interface Sci. 16 , 135 ( 2011 ) . Crossref, Google Scholar
K. E. Sapsford et al. , Anal. Chem. 83 , 4453 ( 2011 ) . Crossref, Google Scholar
N. Wu et al. , Environ. Technol. 2 , 55 ( 2013 ) . Crossref, Google Scholar
M. Yarema , R. Caputo and M. V. Kovalenko , Nanoscale 5 , 8398 ( 2013 ) . Crossref, Google Scholar
S.-Y. Zhang , M. D. Regulacio and M.-Y. Han , Chem. Soc. Rev. 43 , 2301 ( 2014 ) . Crossref, Google Scholar
T. J. Woehl et al. , ACS Nano 6 , 8599 ( 2012 ) . Crossref, Google Scholar
J. Polte et al. , ACS Nano 4 , 1076 ( 2010 ) . Crossref, Google Scholar
M. Harada , N. Tamura and M. Takenaka , J. Phys. Chem. C 115 , 14081 ( 2011 ) . Crossref, Google Scholar
J. Polte et al. , ACS Nano 6 , 5791 ( 2012 ) . Crossref, Google Scholar
I. Olliges-Stadler et al. , Nanoscale 5 , 8517 ( 2013 ) . Crossref, Google Scholar
Y. Sun and Y. Ren , Part. Part. Syst. Charact. 30 , 399 ( 2013 ) . Crossref, Google Scholar