Multicolor Graphene Nanoribbon/Semiconductor Nanowire
Heterojunction Light-Emitting Diodes
†
‡
†
†
†
†
‡
†
‡
Yu Ye, Lin Gan, Lun Dai,*, Hu Meng, Feng Wei, Yu Dai, Zujin Shi, Bin Yu, Xuefeng Guo, and
Guogang Qin *,
†
† State Key Lab for Mesoscopic Physics and School of Physics, Peking University, Beijing 100871, China
‡ College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Abstract
We report novel graphene nanoribbon (GNR)/semiconductor nanowire (SNW)
heterojunction light-emitting diodes (LEDs) for the first time. The GNR and SNW
have a face-to-face contact structure, which has the merit of bigger active region. ZnO,
CdS, and CdSe NWs were employed in our case. At forward biases, the GNR/SNW
heterjunction LEDs could emit light with wavelengths varying from ultraviolet (380
nm) to green (513 nm) to red (705 nm), which were determined by the band-gaps of
the involved SNWs. The mechanism of light emitting for the GNR/SNW
heterojunction LED was discussed. Our approach can easily be extended to other
semiconductor nano-materials. Moreover, our achievement opens the door to
next-generation display technologies, including portable, “see-through”, and
conformable products.
Keywords: graphene; light-emitting diodes; heterojunction; semiconductor nanowire;
Schottky contact
__________________
Corresponding author. E-mail: lundai@pku.edu.cn (L. D.); qingg@pku.edu.cn (G. Q.).
0
1
Introduction
Graphene, a single atomic layer of carbon arranged in a honeycomb lattice, has
attracted a lot of research interest since its discovery in 2004.1-5 Its many important
physical properties, such as high mobility and conductivity,6 high optical
transparency,7 mechanical flexibility8 and robustness,9 and environmental stability,9
have made graphene a promising material in diverse device applications. For example,
it has been used to substitute indium titanium oxide (ITO) as the flexible transparent
conductive electrode for organic photovoltaics10 and light-emitting diodes (LEDs).11
Besides, graphene is considered as a promising alternative for silicon for
next-generation nanoelectronics, especially in producing low-cost, high-efficiency,
lightweight, transparent, and flexible devices.5, 12-16 So far, a lot of work has been
done regarding the above two aspects. However, few work17 aimed at developing the
graphene-based nano-optoelectronic devices, which can be used in next-generation
integratred nano-optoelectronic system, has been reported.
Semiconductor single crystalline nanowires (NWs) can be grown on basically any
substrates, and can be constructed into devices with the bottom-up method.18
Moreover,
they
are
mechanically
flexible
and
robust,
compatible
with
low-temperature device process desired for plastic substrates.19 So far, they have been
used as building blocks in diverse devices, including Si-based optoelectronic
devices,20 and flexible transparent electronic/photonic devices.19, 21
In this letter, we report, for the first time, the graphene nanoribbon
(GNR)/semiconductor NW (SNW) heterojunction LEDs. n-type ZnO, CdS, CdSe
NWs were used in this work. A face-to-face contact structure was employed in the
GNR/SNW heterojunction, which has the merit of larger active area, where the
radiative recombination occurs.22 The emitting light wavelength was determined by
2
the involved SNW. Our achievement can easily be extended to other semiconductor
nano-materials, and has a promising application on transparent flexible devices.
Moreover, our approach pioneers a new realm for graphene-based high efficient
electroluminescence (EL) devices, and even electrically driven lasers, which have
potential application in developing the graphene-based micro/nano-optoelectronic
integration in the future.
Experimental Section
Both the n-type SNWs23-25 and the graphene26 used in this work were synthesized
via the CVD method. Before device fabrication, the graphenes were transferred by the
stamp method with the help of PMMA (poly methyl methacrylate)27 to Si/300 nm
SiO2 substrates for Raman and electrical property characterizations, and transferred to
carbon-coated grids for high-resolution transmission electron microscopy (HRTEM)
characterization (Tecnai F30). Their electrical properties were measured by a
Hall-effect measurement system (Accent HL5500).
The fabrication processes of a GNR/SNW heterojunction LED are as follows
(Figure 1): First, the as-synthesized large-scale graphene was transferred by the stamp
method with the help of PMMA27 to a Si/SiO2 substrate. Second, the SNWs (ZnO,
CdS, or CdSe here) dispersed in ethanol were dropped on the substrate (Figure 1a).
Third, a photoresist pad was patterned to cover one end of a SNW by UV lithography
and development processes (Figure 1b). The photoresist together with the uncovered
SNW was then used as the masks for the following graphene etching process by an
inductively coupled plasma (ICP) etching technique (Figure 1c). Later, the rest
photoresist was removed by acetone, leaving a graphene pad connecting with a GNR
underneath the SNW. Ohmic contact electrodes to the SNW (In/Au (10 /100 nm)) and
graphene pad (Au (60 nm)) were defined by UV lithography followed by thermal
3
evaporation and lift-off processes (Figure 1d). It is worth noting that because an
undercut was form during the oxygen plasma etching process,22 the In/Au electrode
on the SNW will not contact with the GNR to cause a short circuit. The electrical
properties of the GNRs fabricated with the method mentioned above were
investigated by measuring their transistor characteristics. A GNR transistor was
fabricated by using a SiO2 NW as the mask. Here, the insulating SiO2 NW needs not
to be removed before the electrical measurement, and thus avoids additional chemical
or physical processes induced influence to the measured results.
Figure 1. Schematic illustration of the fabrication processes of a GNR/SNW heterojunction LED.
(a). The as-synthesized large-scale graphene was transferred to a Si/SiO2 substrate. After that,
SNW suspension was dropped on the graphene. (b). A photoresist pad was patterned to cover one
end of a SNW by UV lithography and development processes. (c). Oxygen plasma etching was
used to remove the exposed graphene. (d). After removing the photoresist, In/Au and Au ohmic
contact electrodes to SNW and graphene were defined, respectively.
Room-temperature electrical transport properties of the GNR transistors and
GNR/SNW heterojunctions were characterized with a semiconductor characterization
system (Keithley 4200). Raman and EL measurements were done with a microzone
confocal Raman spectroscopy (HORIBA Jobin Yvon, LabRam HR 800) equipped
with a color charge-coupled device (CCD).
Results and discussion
4
The HRTEM and Raman characterization results for the as-synthesized graphenes
(see Supporting Information) demonstrate that the graphenes are high quality and with
monolayer. The typical sheet resistance, hole concentration, and hole mobility of the
graphene obtained by Hall-effect measurement are about 345 Ω/□, 1.84×1014 cm–2,
and 98.6 cm2/V·s, respectively. It is worth noting that the low sheet resistance and
high hole concentration of the as-synthesized graphene guarantee a small series
resistance of the GNR in the heterojunction LED.
Figure 2a shows a field-emission scanning electron microscope (FESEM) image
of an as-fabricated GNR/CdS NW heterojunction LED. We can see that the diameter
of the CdS NW is about 300 nm. During the electrical measurement, the In/Au ohmic
contact electrode of the CdS NW was grounded. The I-V curve (Figure 2b) of the
LED shows an excellent rectification characteristic. An on/off current ratio of
~3.4×107 can be obtained when the voltage changes from +1.5 to –1.5 V. The turn-on
voltage is around 1.1 V. In view of the high hole concentration (1.84×1014 cm–2) and
near-zero band-gap of the GNR,28 the heterojunction structure of the GNR/CdS NW
can be considered approximately as a metal-semiconductor contact of Schottky
model.29, 30 For Schottky barrier diodes made on high-mobility semiconductors such
as ZnO, CdS and CdSe etc., the current I is determined by the thermionic emission of
electrons and can be described by the equation I = I0[exp(eV/nkT)−1],31 where I0 is the
reverse saturation current, e is the electronic charge, V is the applied bias, n is the
diode ideality factor, k is the Bolzmann’s constant, and T is the absolute temperature.
By fitting the measured I-V curve with the above equation, we can obtain n=1.58.
Note that, the GNR/ZnO NW and GNR/CdSe NW heterojunctions show similar
rectification characteristics as above, with the turn-on voltages to be about 0.7 and 1.2
V, respectively.
5
Figure 2. (a) FESEM image of an as-fabricated GNR/CdS NW heterojunction LED. (b)
Room-temperature I-V characteristic of the LED in panel (a) on a semi-log scale. The red straight
line shows the fitting result of the I-V curve by the equation ln(I) = eV/nkT + ln(I0).
Figures 3a-c show the EL images (Olympus BX51M) of the GNR/SNW (ZnO,
CdS, CdSe, respectively) heterojunction LEDs at a forward bias of 5 V. Except for the
ZnO NW case (where the emitting light is invisible ultraviolet light) in Figure 3a,
strong emitting light spots can be seen clearly with naked eyes at the exposed ends of
the NWs. For the CdS NW case (Figure 3b), we can see another glaring light spot on
the NW. This may be due to the scattering from defect or adhered particle on the CdS
NW.32 As the optical image of the graphene on the Si substrate with 600 nm SiO2
layer is unclear, we use the dashed lines to demarcate the graphenes in these figures.
Figures 3d-f show the room-temperature normalized EL spectra at various forward
biases for the GNR/SNW heterojunction LEDs, where the SNWs are ZnO, CdS, and
CdSe NWs, respectively. The EL light collecting points were at the corresponding
exposed ends of the NWs, indicated by the white arrows in the figures. The peak
wavelengths of the EL spectra coincide with the band-edge emission of the involved
SNWs (ZnO, CdS, CdSe, respectively.). This indicates the radiative recombinations of
6
electrons and holes occur in the SNWs. For all the LEDs, EL intensities increase with
the forward biases. Significantly, light emission can be detected at a forward bias as
low as 2.5 V for the CdS NW case. We have studied more than 40 GNR/SNW
heterojunction LEDs. Similar results are obtained.
Figure 3. (a)–(c). The optical images of the GNR/SNW (ZnO, CdS, CdSe, respectively)
heterojunction LEDs at a forward bias of 5 V. Dashed lines were used to demarcate the graphenes
from the substrates. White arrows: the light collecting points during the EL measurements. (d)-(f).
The room-temperature EL spectra for GNR/SNW (ZnO, CdS, CdSe, respectively) heterojunction
LEDs at various forward biases.
Various Si-based SNW heterojunctions have been reported before.20, 23 Lieber’s
group even reported the electrically driven laser from the Si/CdS NW heterojunction
for the first time.33 Compared with Si, graphene has higher conductivity and
mobility,6 which might make the graphene-based SNW heterojunciton superior to its
Si-based rival in realizing high efficient LED or even laser device.
We can qualitatively understand the mechanism of the light emitting for the
GNR/SNW heterojunction LEDs by studying the energy band diagrams. Figure 4a
shows the thermal equilibrium energy band diagram of a graphene/n-type
semiconductor structure, where the work function of graphene is Φ, and the electron
affinity of the semiconductor is χ. Eg, EF correspond to the band-gap and Fermi level
7
of the semiconductor. Due to the difference of their work functions, the energy band
of semiconductor will bend upward at the graphene/semiconductor interface, and the
Fermi levels at the two sides are brought into coincidence after contacting. Under a
forward bias (i.e. a positive bias on graphene), the built-in potential is lowered.
Therefore, more electrons will flow from n-type semiconductor to graphene, and
simultaneously, more holes will flow from graphene to n-type semiconductor. Herein,
as the SNW is the direct band-gap semiconductor, which has higher electron-hole
radiative recombination rate, the injected holes and electrons will mainly recombine
in the SNW region. Therefore, the corresponding EL spectrum will be determined by
the band-edge emission of the SNW. It is worth noting that, in our face-to-face contact
LED, the active region, where the radiative recombination occurs, is bigger, compared
to the crossed nanowire structure reported previously.20, 23, 34, 35 This merit would
benefit the realization of high efficient EL, or even electrically driven laser of
GNR/SNW heterojunction in future. For this purpose, further optimization of the
SNW and graphene materials, the device structure, as well as the fabrication processes
is ongoing.
Figure 4. Schematic illustration of the energy band diagram of a graphene/semiconductor
heterojunction. (a) The thermal equilibrium energy band diagram. (b) The energy band diagram of
8
the heterojunction under a forward bias. Φ: the work function of graphene; χ: the electron affinity
of the semiconductor.
Conclusion
We have fabricated and studied GNR/SNW heterojunction LEDs for the first time.
The GNR and SNW have a face-to-face contact structure, which has the merit of
bigger active region. ZnO, CdS, and CdSe NWs were employed in our case. The
emitting light wavelengths of the as-fabricated LEDs are determined by the band-gaps
of the involved SNWs in the GNR/SNW heterojunctions, which can vary from
ultraviolet (380 nm) to green (513 nm) to red (705 nm). Our accomplishment will
benefit in developing graphene-based multicolor display, optoelectronic integration,
and even electrically driven laser in future.
Acknowledgement
This work was supported by the National Natural Science Foundation of China
(Nos.10774007, 11074006, 10874011, 50732001), the National Basic Research
Program of China (Nos. 2007CB613402), and the Fundamental Research Funds for
the Central Universities.
9
References
(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S.
V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666-669.
(2) Lin, Y. M.; Jenkins, K. A.; Valdes-Garcia, A.; Small, J. P.; Farmer, D. B.; Avouris,
P. Nano Lett. 2009, 9, 422-426.
(3) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.;
Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Science 2007, 315, 1379.
(4) Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. C. Nature Photonics 2010, 4,
611-622.
(5) Geim. A. K.; Novoselov, K. S. Nature Mater. 2007, 6, 183-191.
(6) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone, J.; Kim, P.;
Stormer, H. L. Solid State Commun. 2008, 146, 351-355.
(7) Li, X. S.; Zhu, Y. W.; Cai, W. W.; Borysiak, M.; Han, B. Y.; Chen, D.; Piner, R. D.;
Colombo, L.; Ruoff, R. S. Nano Lett. 2009, 9, 4359-4363.
(8) Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Science 2008, 321, 385-388.
(9) Briggs, B. D.; Nagabhirava, B.; Rao, G.; Geer, R.; Gao, H. Y.; Xu, Y.; Yu, B. Appl.
Phys. Lett. 2010, 97, 223102.
(10) de Arco, L. G.; Zhang, Y.; Schlenker, C. W.; Ryu, K.; Thompson, M. E.; Zhou, C.
W. ACS Nano 2010, 4, 2865-2873.
(11) Wu, J. B.; Agrawal, M.; Becerril, H. A.; Bao, Z. N.; Liu, Z. F.; Chen, Y. S.;
Peumans, P. ACS Nano 2010, 4, 43-48.
(12) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.;
10
Kim, P.; Choi, J. Y.; Hong, B. H. Nature 2009, 457,706-710.
(13) Zheng, Y.; Ni, G. X.; Toh, C. T.; Zeng, M. G.; Chen, S. T.; Yao, K.; Özyilmaz, B.
Appl. Phys. Lett. 2009, 94, 163505.
(14) Zhou, Y.; Loh, K. P. Adv. Mater. 2010, 22, 3615-3620.
(15) Berger, C.; Song, Z. M.; Li, T. B.; Li, X. B.; Ogbazghi, A. Y.; Feng, R.; Dai, Z. T.;
Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. J. Phys. Chem. B 2004,
108, 19912-19916.
(16) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayou, D.;
Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A.
Science 2006, 312, 1191-1196.
(17) Lee, J. M.; Choung, J. W.; Yi, J.; Lee, D. H.; Samal, M.; Yi, D. K.; Lee, C. H.; Yi,
G. C.; Paik, U.; Rogers, J. A.; Park, W. I. Nano Lett. 2010, 10, 2783-2788.
(18) Yang, P. D.; Yan, R. X.; Fardy, M. Nano Lett. 2010, 10, 1529-1536.
(19) Ju, S.; Facchetti, A.; Xuan, Y.; Liu, J.; Ishikawa, F.; Ye, P. D.; Zhou, C. W.; Marks,
T. J.; Janes, D. B. Nature Nanotech. 2007, 2, 378-384.
(20) Huang, Y.; Duan, X. F.; Lieber, C. M. Small 2005, 1, 142-147.
(21) Kim, D. H.; Ahn, J. H.; Choi, W. M.; Kim, H. S.; Kim, T. H.; Song, J. Z.; Huang,
Y. Y.; Liu, Z. J.; Lu, C.; Rogers, J. A. Science 2008, 320, 507-511.
(22) Liu, C.; Dai, L.; Ye, Y.; Sun, T.; Peng, R. M.; Wen, X. N.; Wu, P. C.; Qin, G. G. J.
Mater. Chem. 2010, 20, 5011-5015.
(23) Yang, W. Q.; Huo, H. B.; Dai. L.; Ma, R. M.; Liu, S. F.; Ran, G. Z.; Shen, B.; Lin,
C. L.; Qin, G. G. Nanotechnology 2006, 17, 4868-4872.
11
(24) Ye, Y.; Dai, L.; Wen, X. N.; Wu, P. C.; Pen, R. M.; Qin, G. G. ACS Appl. Mater.
Interfaces 2010, 2, 2724-2727.
(25) Liu, C.; Wu, P. C.; Sun, T.; Dai, L.; Ye, Y.; Ma, R. M.; Qin, G. G. J. Phys. Chem.
C 2009, 113, 14478-14481.
(26) Ye, Y.; Gan, L.; Dai, L.; Guo, X. F.; Meng, H.; Yu, B.; Shi, Z. J.; Shang, K. P.;
Qin, G. G. Nanoscale 2011, DOI: 10.1039/c0nr00999g.
(27) Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H.; Bulovic, V.; Dresselhaus, M. S.;
Kong, J. Nano Lett. 2009, 9, 30-35.
(28) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K.
Rev. Mod. Phys. 2009, 81, 109-162.
(29) Thomas, D.; Boettcher, J.; Burghard, M.; Kern, K. Small 2010, 6, 1868-1872.
(30) Li, X. M.; Zhu, H. W.; Wang, K. L.; Cao, A, Y.; Wei, J. Q.; Li, C. Y.; Jia, Y.; Li,
Z.; Li, X.; Wu, D. H. Adv. Mater. 2010, 22, 2743-2748.
(31) Sharma, B. L. Metal-Semiconductor Schottky Barrier Junctions and Their
Applications; Plenum Press: New York, 1984.
(32) Ma, R. M.; Wei, X. L.; Dai, L.; Liu, S. F.; Chen, T.; Yue, S.; Li, Z.; Chen, Q.; Qin,
G. G. Nano Lett. 2009, 9, 2679-2703.
(33) Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241-245.
(34) Zhong, Z. H.; Qian, F.; Wang, D. L.; Lieber, C. M. Nano Lett. 2003, 3, 343-346.
(35) Gudiksen, M. S.; Lauhon, L. J.; Wang, J. F.; Smith, D. C.; Lieber, C. M. Nature
2002, 415, 617-620.
12