Integration of colloidal silicon nanocrystals on metal electrodes in single-electrontransistor
Yasuhiro Higashikawa, Yasuo Azuma, Yutaka Majima, Shinya Kano, and Minoru Fujii

Citation: Appl. Phys. Lett. 109, 213104 (2016); doi: 10.1063/1.4968583
View online: http://dx.doi.org/10.1063/1.4968583
View Table of Contents: http://aip.scitation.org/toc/apl/109/21
Published by the American Institute of Physics

APPLIED PHYSICS LETTERS 109, 213104 (2016)

Integration of colloidal silicon nanocrystals on metal electrodesin single-electron transistor
Yasuhiro Higashikawa,1 Yasuo Azuma,2 Yutaka Majima,2 Shinya Kano,1,a) and Minoru Fujii1
1

Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University,
Kobe 657-8501, Japan
2
Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama 226-8503, Japan(Received 7 September 2016; accepted 11 November 2016; published online 22 November 2016)
We develop a facile process to integrate colloidal silicon nanocrystals (Si NCs) with metal electrodes
in a single-electron transistor by self-assembly. Gold (Au) surface is modified by an amineterminated self-assembled monolayer to have a positive potential. All-inorganic boron (B) and phosphorus (P) codoped Si NCs, with a negative surface potential and size-controllability, are selectively
adsorbed on an amine-terminated Au surface by electrostatic attraction. We demonstrate the fabrication of SETs consisting of electroless-plated Au nanogap electrodes and codoped Si NCs using thisprocess and observation of clear Coulomb diamonds at 9 K. Published by AIP Publishing.
[http://dx.doi.org/10.1063/1.4968583]

Colloidal semiconductor nanocrystals (NCs) have been
intensively studied because their optical and electronic properties are largely modified by quantum confinement with respect
to the bulk counterparts.1 They can be a precursor for thedevelopment of optoelectronic devices by the solution-based
processes.2 High-performance field-effect transistors,3,4 photovoltaics,5,6 and light-emitting diodes7,8 have been developedby using the spin-coated colloidal semiconductor NCs.
One of the promising applications of colloidal NCs is asingle-electron transistor (SET), which can be used in several
kinds of nanodevices such as reconfigurable Boolean logic,9,10
single-electron charge sensor,11,12 and single-electron pump.13
Since the operational temperature of SETs is strongly related
to the size of a Coulomb island, size-regulated NCs are potentially very attractive as a material for a Coulomb island.14–16
For this application, single-nanometer NCs are required to be
immobilized between the nanogap electrodes, which is notpossible by conventional semiconductor technology. Selfassembly with the assistance of organic molecules is an effective route to immobilize NCs in the nanogap electrodes.
The most widely used NC material in SETs is gold (Au).
By using the well-established process utilizing the alkanedithiol molecules, Au NCs can be bound onto metal electrodes.17–20 Furthermore, colloidal II-VI semiconductor NCs
such as cadmium selenide (CdSe) NCs21 are used for the fabrication of SETs and a light-emitting SET.22 However, NCs
composed of these heavy-metal elements are not compatible
with the existing complementary metal-oxide-semiconductor(CMOS) technology. Integration of SETs with these colloidal
NCs in the existing electronic devices is not straightforward.
In this work, we develop a process to integrate colloidal
silicon (Si) NCs with SETs. Si is an environmental-friendly
and CMOS-compatible element. However, processes toimmobilize individual Si NCs on metal electrodes are notestablished. The reason is that it is difficult to use the alkanedithiol molecules for immobilizing a Si NC on metal surface.
a)Author to whom correspondence should be addressed. Electronic mail:kano@eedept.kobe-u.ac.jp0003-6951/2016/109(21)/213104/5/$30.00

This is due to the fast oxidation of the surface and the inability of the oxidized surface, which makes it difficult to form achemical bond between Si surface and a thiol group.23
We propose an essential technique to immobilize Si
NCs on metal surface based on electrostatic attraction. In
this work, we employ boron (B) and phosphorus (P) codoped
colloidal Si NCs (codoped Si NCs).24–26 Codoped Si NCs
have heavily B and P codoped shells, which induces a negative potential on the surface (f-potential: 42– 27 mV)24
and prevents the agglomeration in polar solvents by the
Coulomb repulsion. The perfect dispersion of NCs in solution is crucial for the self-assembly onto metal surface. The
Codoped Si NCs are attached on positively charged Au surface with an amine-terminated self-assembled monolayer(SAM) by electrostatic attraction. We demonstrate the formation of a SET by placing the codoped Si NCs in the Au
nanogap electrodes.
Codoped Si NCs were prepared using a method describedin the previous papers.24–26 Borophosphosilicate glass films
were deposited on a stainless steel plate by co-sputtering Si,
SiO2, B2O3, and P2O5. The films were removed from thestainless steel plate and the films were crushed in a mortar.
The powder was annealed in a N2 atmosphere at 1200  C for
30 min to grow codoped Si NCs. The annealed powder was
dissolved in a hydrofluoric acid solution for 30 min in an ultrasonic bath to extract codoped Si NCs. Codoped Si NCs were
centrifuged and transferred into methanol. The concentrationof Si in the solution was 0.5–0.7 mg/ml. The obtained NC
solution was slightly acidic (pH ¼ 4–6).
Figure 1(a) shows a transmission electron microscope(TEM) image of codoped Si NCs. Because a negative surface
potential prevents codoped Si NCs from aggregation,24 NCs
are isolated one by one and no aggregate is observed. The
inset image is a high-resolution TEM (HRTEM) image of aNC. The lattice fringe corresponding to the {111} plane of
Si crystal is observed.25 Figure 1(b) shows the size distribution of codoped Si NCs obtained from the TEM images.
Average diameter of NCs (Dave) is 8.0 nm with a standard
deviation (r) of 2.0 nm. Note that Dave can be controlled in a

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Appl. Phys. Lett. 109, 213104 (2016)

FIG. 1. (a) TEM image and (b) size distribution of Si NCs annealed at
1200  C. Inset: HRTEM image of Si NC. (c) Schematic illustration of amethod to immobilize Si NCs electrostatically on Au surface.
24

wide range (1–14 nm) by changing the growth temperature.
Irrespective of the size, codoped Si NCs have negative surface potential and thus the process can be applied to codoped
Si NCs with different sizes. In this study, we chose 8.0 nm
diameter NCs to make observations of individual NCs on
metal electrodes by using scanning electron microscopy
easier.
A self-assembly method to immobilize the codoped Si
NCs on Au electrodes is described in Figure 1(c). We deposited a titanium (Ti) and Au film on a thermally oxidized SiO2/
Si substrate. After cleaning the substrate by a UV/O3 cleaner(Samco, UV-1), the substrate was immersed in 6-amino-1hexanethiol [HS(CH2)6NH2, Dojindo, Japan] solution in ethanol (1 mM) for 24 h to form a self-assembled monolayer(SAM) on Au surface. Subsequently, the SAM-treated substrate was immersed in Si NC solution. The concentration of
Si NC solution was diluted 1000 times by methanol. Because
original Si NC solution was acidic, amino groups in a
6-amino-1-hexanethiol SAM were protonated and Au surface
was positively charged in Si NC solution.27,28 Therefore, negatively charged codoped Si NCs were attracted by Coulombinteraction and adsorbed on Au surface. Finally, the substrate
was rinsed in methanol and distilled water and then dried byN2 gas.
Figure 2 shows an infrared (IR) absorption spectrum of a6-amino-1-hexanethiol SAM on Au surface by reflectionabsorption spectroscopy measurement (PerkinElmer, Spectrum
GX). In the spectrum, two prominent absorption peaks areobserved at 2860 cm1 and 2928 cm1 assigned to the symmetric and asymmetric CH2 stretching modes of a hydrocarbon
chain, respectively.29 A weak absorption peak at 3408 cm1 is
assigned to the NH stretching mode.29 From these results, we
confirmed that a 6-amino-1-hexanethiol SAM is formed on Au
surface.
To confirm the validity of the method in Figure 1(c), we
prepared two substrates. One is a 6-amino-1-hexanethiol
SAM-treated Au substrate and the other is a bare Au substrate
as a control sample. Figures 3(a) and 3(b) show the SEM(HITACHI, SU-8000) images of a 6-amino-1-hexanethiol
SAM-treated Au substrate and a bare Au substrate after

FIG. 2. IR absorption spectrum of a 6-amino-1-hexanethiol SAM on Au
surface.

adsorption of Si NCs, respectively. In Figure 3(a), a large
number of Si NCs represented by white spots are observed on
SAM-treated Au surface. Each Si NC is well isolated even on
Au surface because of electrostatic repulsion during immersion in solution. On the other hand, only a few Si NCs areadsorbed on bare Au surface. Thus, codoped Si NCs can beelectrostatically adsorbed on Au surface via a 6-amino-1-hexanethiol SAM. Figure 3(c) shows a SEM image of boundaryof SAM-treated Au and SiO2 surface after immersion in Si
NC solution. In contrast to SAM-treated Au surface, Si NCs
are not observed on SiO2 surface because a 6-amino-1-hexanethiol SAM is not assembled on SiO2 surface. According to
the above result, codoped Si NCs can be selectively assembled

FIG. 3. (a) SEM image of a 6-amino-1-hexanethiol SAM-treated Au substrate and (b) a bare Au substrate after immersion in Si NC solution.
Immersion time of both substrates was 1 h. (c) SEM image of boundary of
SAM-treated Au and SiO2 surface after immersion in Si NC solution repeatedly. Total immersion time was 4.5 h.

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on Au surface by the method described in Figure 1(c). Notethat a thiol group can effectively attach on many metal surface,30 and thus, this procedure can be used for other metal
surface as well as Au.
We fabricated a SET by using codoped Si NCs, a6-amino-1-hexanethiol SAM, and electroless Au plated nanogap electrodes. Initial Ti (2 nm)/Au (10 nm) nanogap electrodes were fabricated on a Si substrate with 300-nm thermally
oxidized SiO2 by electron beam lithography and photolithography. The number of nanogap electrodes was in total 49.
Initial nanogap separation between source and drain electrodes was 25 nm and two side-gate electrodes were also fabricated. Nanogap separation was reduced to the size of NCs byelectroless Au plating. A detailed process of electroless Au
plating was reported in a previous paper.31 After fabricationof Au nanogap electrodes, the same adsorption process of Si
NCs in Figure 1(c) was carried out.
Figure 4(a) shows a SEM image of a fabricated device
and a circuit for electrical measurement. In this study, only
one side-gate is used for the SET operation (the top electrodein Fig. 4(a)). Si NCs bridge between source and drain electrodes. Electrical measurements were performed by using
a semiconductor parameter analyzer (Agilent, B1500) anda probe station (Nagase Techno-Engineering, GRAIL
10-LOGOS01S) in vacuum (105 Pa) at 9 K. Figure 4(b)shows measured drain current–drain voltage (ID–VD) characteristics under applying different gate voltages VG ¼ 1.92

FIG. 4. (a) SEM image of a fabricated device with a circuit for electricalmeasurement. Immersion time of the Au nanogap electrodes in Si NC solution was 3 s. The bottom electrode in the image was not used in this work.
(b) ID–VD characteristics of a fabricated SET under different gate voltages
VG ¼ 1.92 and 0 V.

Appl. Phys. Lett. 109, 213104 (2016)

and 0 V. In both gate voltages, a nonlinear characteristic and a
voltage range where drain current is suppressed are observed.
The suppressed current region is modulated by the gate voltages; therefore, the Coulomb blockade occurs because Si
NCs between the nanogap electrodes work as a Coulomb
island.
Figure 5(a) shows the measured drain current–gate voltage (ID-VG) characteristics under applying different drain
voltages. The oscillation of drain current by gate voltage(Coulomb oscillation) is clearly observed. Note that aperiodic oscillation indicates that several Si NCs work as a
Coulomb island in a SET, in contrast to the periodic oscillation of a metal single-dot SET.9 Figure 5(b) shows differential conductance (dID/dVD) plotted as a function of drain
voltage and gate voltage (stability diagram). The stability
diagram is obtained corresponding to the Coulomb gap in
ID-VD characteristics (Fig. 4(b)) and the peak separation of
ID-VG characteristics (Fig. 5(a)). Measured complex stability
diagram also indicates that several Si NCs are adsorbed
between source and drain electrodes. Note that the rhombic
shape of Coulomb diamonds guarantees a single NC working as a Coulomb island.9 A network of dopants in NCs
may cause the complex stability diagram; however, we do
not have any evidence to support this assumption by now. It
should be stressed here that, in the nanogap electrodes without NCs, no detectable current is observed (see supplementary material). On the other hand, with NCs, we observe
SET operations in 3 nanogap electrodes out of 49. The yieldis thus 6%.
Here, we roughly estimate a charging energy of single
NC to confirm a validity of the operation of SET at 9 K. In

FIG. 5. (a) ID-VG characteristics of a fabricated SET under applying different drain voltage VD ¼ 0.15, 0.10, 0.05, 0.05, 0.10 and 0.15 V (fromtop to bottom). (b) Stability diagram of a fabricated SET.

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Appl. Phys. Lett. 109, 213104 (2016)

the case of a SET consisting of electroless plated nanogap
electrodes and a single NC, a total capacitance CR is wellevaluated by using a concentric sphere model16 (i.e., a single
NC is almost shielded with metal electrodes)
0
11
1
1
CR ¼ 4p0 r @ 
A ;
D D
þd
2 2

(1)

where 0 is the permittivity of vacuum, r is a relative permittivity of the space, D is a diameter of a NC, and d is an average distance between a single NC and metal surface.
Expected total capacitance is 5.0 aF according to this concentric sphere model. In this calculation, we assume r is a relative permittivity of the alkanethiol (r ¼ 2.6), D is 8.0 nm, andd is a length of 6-amino-1-hexanethiol. The value of d isassumed to be 1.2 nm, which is a similar length of 1,6-hexanedithiol.21 By calculating CR in Eq. (1), we evaluate a charging
energy (Ec) as Ec ¼ e2 =CR , where e is the elementarycharge.32 Evaluated Ec is 32 meV and this value is muchlarger than thermal energy at 9 K (0.78 meV). Therefore, anoperation of a SET using codoped Si NCs (Dave ¼ 8.0 nm) at
9 K is reasonable.
In our previous study, the B concentration of codoped Si
NCs is always larger than P concentration.24 Analyzed byvalence-band spectroscopy, the Fermi level of codoped Si
NCs larger than 4 nm is close to the highest occupied molecular orbital (HOMO) level.33 This result indicates that 8-nm
codoped Si NCs are p-type semiconductor. In general, a transistor with a p-type quantum dot works in a negative gate
voltage region (i.e., single-hole transistor34,35). However, theobserved Coulomb diamond in Fig. 5(b) is in both positive
and negative gate voltage regions, which is similar to that ofmetal NCs. A possible reason is that the Fermi level ofcodoped Si NCs lies among the B acceptor levels, similar todegenerate semiconductors.36 In fact, the P and B concentrations in the present Si NCs are several at. % and about 10
at. %, respectively,24 which are much higher than the solid
solubility in Si crystal.37 Although a majority of them areconsidered to be inactive and accumulated on or near thesurface, it is still very plausible that the NCs are degenerate
semiconductors. Unoccupied B states also exist above theFermi level as well as thermally activated (occupied) B
states, and thus, codoped Si NCs can have a similar property to metal NCs. If the size of codoped Si NCs is reduced
less than 3 nm, the Fermi level approaches center of theenergy gap and a typical property of a semiconductor NC is
expected.
In conclusion, we developed a process to integrate colloidal Si NCs with metal surface in SETs by self-assembly.
The method uses electrostatic attraction between negatively
charged B and P codoped Si NCs and positively charged
amine-terminated Au surface. We demonstrated the operation of the fabricated SETs at 9 K. The method developed inthis work is versatile for the fabrication of different kinds ofNC-based nanodevices by a bottom-up process.
See supplementary material for the current–voltage
characteristics of electroless plated nanogap electrodes without Si NCs.

We would like to thank Ms. M. Miyakawa for the SEM
observation and Dr. H. Sugimoto for the HRTEM
observation. This work was supported by the Collaborative
Research Project of Laboratory for Materials and Structures(Tokyo Institute of Technology); 2015 JST Visegrad Group(V4)-Japan Joint Research Project on Advanced Materials“NaMSeN”; Hyogo Science and Technology Association;
Kyoto Technoscience Center; JSPS KAKENHI Grant No.
JP16H03828; MEXT Elements Strategy Initiative to Form
Core Research Center from the Ministry of Education,
Culture, Sports, Science, and Technology (MEXT), Japan;
and the BK Plus program, Basic Science Research program(NRF-2014R1A6A1030419).
1

A. Zabet-Khosousi and A.-A. Dhirani, Chem. Rev. 108, 4072 (2008).
D. V. Talapin, J.-S. Lee, M. V. Kovalenko, and E. V. Shevchenko, Chem.
Rev. 110, 389 (2010).
3
D. S. Dolzhnikov, H. Zhang, J. Jang, J. S. Son, M. G. Panthani, T. Shibata,S. Chattopadhyay, and D. V. Talapin, Science 347, 425 (2015).
4
J.-H. Choi, H. Wang, S. J. Oh, T. Paik, P. S. Jo, J. Sung, X. Ye, T. Zhao,
B. T. Diroll, C. B. Murray, and C. R. Kagan, Science 352, 205 (2016).
5
J. Tang, K. W. Kemp, S. Hoogland, K. S. Jeong, H. Liu, L. Levina, M.
Furukawa, X. Wang, R. Debnath, D. Cha, K. W. Chou, A. Fischer, A.
Amassian, J. B. Asbury, and E. H. Sargent, Nat. Mater. 10, 765 (2011).
6
M. V. Kovalenko, Nat. Nanotechnol. 10, 994 (2015).
7
E. H. Gong, X. Yang, G. Walters, R. Comin, Z. Ning, E. Beauregard, V.
Adinolfi, O. Voznyy, and E. H. Sargent, Nat. Photonics 10, 253 (2016).
8
F. Maier-Flaig, J. Rinck, M. Stephan, T. Bocksrocker, M. Bruns, C.
K€ubel, A. K. Powell, G. A. Ozin, and U. Lemmer, Nano Lett. 13, 475(2013).
9
K. Maeda, N. Okabayashi, S. Kano, S. Takeshita, D. Tanaka, M.
Sakamoto, T. Teranishi, and Y. Majima, ACS Nano 6, 2798 (2012).
10
S. K. Bose, C. P. Lawrence, Z. Liu, K. S. Makarenko, R. M. J. vanDamme, H. J. Broersma, and W. G. van der Wiel, Nat. Nanotechnol. 10,1048 (2015).
11
K. Horibe, T. Kodera, and S. Oda, Appl. Phys. Lett. 106, 83111 (2015).
12
S. Kano, Y. Azuma, D. Tanaka, M. Sakamoto, T. Teranishi, L. W. Smith,
C. G. Smith, and Y. Majima, J. Appl. Phys. 114, 223717 (2013).
13
A. Fujiwara, K. Nishiguchi, and Y. Ono, Appl. Phys. Lett. 92, 042102(2008).
14
Y. Azuma, Y. Yasutake, K. Kono, M. Kanehara, T. Teranishi, and Y.
Majima, Jpn. J. Appl. Phys., Part 1 49, 90206 (2010).
15
N. Okabayashi, K. Maeda, T. Muraki, D. Tanaka, M. Sakamoto, T.
Teranishi, and Y. Majima, Appl. Phys. Lett. 100, 33101 (2012).
16
S. Kano, D. Tanaka, M. Sakamoto, T. Teranishi, and Y. Majima,
Nanotechnology 26, 45702 (2015).
17
Y. Azuma, Y. Onuma, M. Sakamoto, T. Teranishi, and Y. Majima,
Nanoscale 8, 4720 (2016).
18
Y. Noguchi, M. Yamamoto, H. Ishii, R. Ueda, T. Terui, K. Imazu, K.
Tamada, T. Sakano, and K. Matsuda, Jpn. J. Appl. Phys., Part 1 52,
110102 (2013).
19
S. Kano, Y. Azuma, K. Maeda, D. Tanaka, M. Sakamoto, T. Teranishi, L.
W. Smith, C. G. Smith, and Y. Majima, ACS Nano 6, 9972 (2012).
20
S. Kano, T. Tada, and Y. Majima, Chem. Soc. Rev. 44, 970 (2015).
21
D. L. Klein, R. Roth, A. K. L. Lim, A. P. Alivisatos, and P. L. McEuen,
Nature 389, 699 (1997).
22
M. S. Gudiksen, K. N. Maher, L. Ouyang, and H. Park, Nano Lett. 5, 2257
(2005).
23
S. A. Claridge, W. Liao, J. C. Thomas, Y. Zhao, H. H. Cao, S. Cheunkar,
A. C. Serino, M. Andrews, and P. S. Weiss, Chem. Soc. Rev. 42, 2725
(2013).
24
H. Sugimoto, M. Fujii, K. Imakita, S. Hayashi, and K. Akamatsu, J. Phys.
Chem. C 117, 11850 (2013).
25
H. Sugimoto, M. Fujii, K. Imakita, S. Hayashi, and K. Akamatsu, J. Phys.
Chem. C 116, 17969 (2012).
26
H. Sugimoto, M. Fujii, K. Imakita, S. Hayashi, and K. Akamatsu, J. Phys.
Chem. C 117, 6807 (2013).
27
T. Zhu, X. Fu, T. Mu, J. Wang, and Z. Liu, Langmuir 15, 5197 (1999).
28
F. Kuemmeth, K. I. Bolotin, S.-F. F. Shi, and D. C. Ralph, Nano Lett. 8,4506 (2008).
29
N. B. Colthup, L. H. Daly, and S. E. Wiberley, Introduction to Infrared
and Raman Spectroscopy (Academic Press, 1964).
2

213104-5
30

Higashikawa et al.

A. Ulman, Chem. Rev. 96, 1533 (1996).
V. M. Serdio, V. Y. Azuma, S. Takeshita, T. Muraki, T. Teranishi, and Y.
Majima, Nanoscale 4, 7161 (2012).
32
L. P. Kouwenhoven, N. C. van der Vaart, A. T. Johnson, W. Kool, C. J. P.
M. Harmans, J. G. Williamson, A. A. M. Staring, and C. T. Foxon, Z.
Phys. B 85, 367 (1991).
33
Y. Hori, S. Kano, H. Sugimoto, K. Imakita, and M. Fujii, Nano Lett. 16,2615 (2016).
31

Appl. Phys. Lett. 109, 213104 (2016)34

S. Lee, K. Miyaji, M. Kobayashi, and T. Hiramoto, Appl. Phys. Lett. 92,
73502 (2008).
J. Llobet, E. Krali, C. Wang, J. Arbiol, M. E. Jones, F. Perez-Murano, andZ. A. K. Durrani, Appl. Phys. Lett. 107, 223501 (2015).
36
D. Mocatta, G. Cohen, J. Schattner, O. Millo, E. Rabani, and U. Banin,
Science 332, 77 (2011).
37
S. M. Sze, Physics of Semiconductor Devices, 2nd ed. (Wiley, New York,
1981).
35