genezated graphene (graphane) [7]. In conventional materials, the band gap is fixed by their crystalline structure, preventing such band gap con trol.
XI INTERNATIONAL CONFERENCE AND SEMINAR EDM'2010, SECTION I, JUNE 30 - JUL Y 4, ERLAGOL
74
Electrical Properties ofNanocomposite Graphene-Organic Monolayers 2 2 2 i2 Igor A. Kotin , , Irina V. Antonova , Regina A. Soots , Victor Ya. Prinz i Novosibirsk State Technical University, Novosibirsk, Russia 2 Institute of Semiconductor Physics, SB RAS, Novosibirsk, Russia Abstract: The possibility to create the nanocomposite layers
temperature in the range of 120°C-200°C causes to in
based on graphene and organic monolayers is demonstrated in the report. Conductivity of the nanocomposite layers is found to strongly vary depending upon the temperature used during fabrication process. The opening of band gap for these layers under some fabrication regimes is also found.
crease in nanocomposite resistance up to factor 103 and
Index Terms
cludes the following steps:
-
Graphene, organic monolayer, conductivity,
band gap.
RAPHENE
G
leads to opening of a band gap.
II. EXPERIMENTAL Fabrication process of the nanocomposite layers in (1) An electrostatic exfoliation of the graphene layers [8] and transfer it on the 300
nm
Si02 / Si substrate. We
have obtained the graphene flakes with the size up to
I. INTRODUCTION
50xl00 Om and the thickness about d � 3
IS ATTRACTED a lot of attention due
to its unique electronic properties and exiting poten
tials in post-silicon electronics. The band structure of the graphene monolayer strongly depends on its surface cha racteristics or the underlying substrate. Chemical functio nalization of this two-dimensional nanoobject is one of the main methods to manipulate with a type and a level of the graphene conductivity [1]. Functionalization is often provided by deposition of some monolayer on the gra phene [2] or between grapheme layers and substrate [3]. This widens the range of electronic, chemical, mechanical and other properties achievable in graphene-based mate rials and makes them more suitable for specific applica tions. An important step toward making graphene useful as a semiconductor is opening of a band gap due to sur face functionalization. For an oxygen-terminated surface, the graphene exhibits a fmite energy band gap, while the band gap is closed when the oxygen atoms on the sub strate are passivated with hydrogen atoms [4]. An epitaxi al film of graphene on a silicon carbide substrate results in
(2)
An
intercalation
of
the
nm .
N-methylpyrrolidone,
CsH9NO. A choose of intercalation type is based on the well known property of the N-methylpyrrolidone to pene trate between every graphene layer leading to fabrication of the uniform nanocomposite layers. Thickness of 3 flake after intercalation became �4
nm
nm .
(3) Treatment of different samples at temperature in the range of 125°C -250°C for 10 min with the step of 25°C. Fig. 1 demonstrates the schema and images of nano composite layer on Si02 / Si substrate with two Ag con tacts (Ag organic paste). Such structures can be measured in a diode configuration and in a transistor one with use of the substrate as a gate electrode. The reference samples were the graphene flakes with a similar thickness created by the same electrostatic exfol iation and with the similar Ag contacts. The thickness of flakes was determined with use of atomic force microsco py. Current - voltage (I-U) characteristics were measured using the Keithley picoampermeter 6485 in temperature range of 80-300 K.
a band gap of 0.26 eV [5]. It was demonstrated the realiza
tion ofa tunable electronic band gap up to 0.25 eV in electri cally gated bilayer graphene [6] and up to 4.9 eV in hydro genezated graphene (graphane) [7]. In conventional materials, the band gap is fixed by their crystalline structure, preventing such band gap con trol. The electronic band gap is an intrinsic property of semiconductors and insulators that largely determines their transport and optical properties. As such, it has a central role in modem device physics and technology and governs the operation of semiconductor devices. A tuna ble band gap would be highly desirable because it would allow great flexibility in design and devices optimization.
Fig. 1. Schema and images of nanocomposite layer on Si02 / Si substrate with two Ag contacts. The right figure was obtained by means of optical microscopy and has size of nanocomposite flake 6 x 36 Om.
In the present report we found a new graphene functiona lization coating, which allows us to manipulate with con ductivity and band gap of nanocomposite by means of
III. RESULTS AND DISCUSSION
temperature used during fabrication process. Variation of ISBN 978-1-4244-6628-3/10/$26.00 © IEEE
KOTIN et al.: ELECTRICAL PROPERTIES OF NANOCOMPOSITE GRAPHENE ...
75
TABLE I
Figures 2a, b present the I-V characteristics for nano composite layers annealed at temperature 125°C and 150°C. I-V-curve for reference structure annealed at tem perature 150°C is given in Fig. 2c. I-V characteristics for all reference samples were linear and do not depend on
RESISTIVITY p* FOR STRUSTURS CREATED AT DIFFERENT TEMPERATURES Sample
temperature in the rage of 80-300K. A typical sheet resis tivity p* (p*was determined as usual resistivity p divided on the layer thickness d) of all reference samples (see
Nanocomposite Layer
Table 1) was equal to 1-2 KOhm. I-V curves for nano composite layers are non-linear with strong temperature dependence. The sheet resistivity p* for the nanocompo site layers created at different temperatures are given in Table 1. One can see that the nanocomposite layers dem onstrate high resistivity and p-type conductivity (this statement follows from shift of the I-VG curve measured versus gate voltage to positive values).
Graphene
T,oC
p*, KOhm
125
10· ... 108
150
10" ... 10
175.
105 ... 10"
200
10' ... 10"
225
107 ... 108
250
107 ... 108
125-250
1 ... 2
Based on data Fig.2a, b we can tell about the opening of the band gap, which depends on the temperature used for the nanocomposite fabrication. For the structures with maximal resistivity created at 150°C the band gap value can be estimated as 3 eV. The measurements oftunneling current between the nanocomposite layer and probe (STM regime) also support this statement. Additional annealing at T
=
350°C for 5 min of the na
nocomposite layer fabricated at T :S 150°C leads to de crease in resistivity about one order of magnitude. For the layers created at high temperature resistivity is not pro nouncedly changed. Our results clearly demonstrate that the interaction be tween the graphene and N-methylpyrrolidone during an nealing in temperature range of 125°C-250°C leads to �
�
4
�
0 2 U, V
10"
4
6
formation of new nanocomposite material with the high
8
resistivity and the band gap. Temperature of 350°C is not high enough for decomposition of this material.
b
IV. CONCLUSION
lO,s 10'·
Intercalation of N-methylpyrrolidone in the few layers
-
graphene structure with subsequent anneal at 125°C250°C leads to formation of the new nanocomposite ma
