Download Band Mapping Across a pn-Junction in a Nanorod by Scanning

Letter
pubs.acs.org/NanoLett
Band Mapping Across a pn-Junction in a Nanorod by Scanning
Tunneling Microscopy
Abhijit Bera, Sukumar Dey, and Amlan J. Pal*
Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
S Supporting Information
*
ABSTRACT: We map band-edges across a pn-junction that was formed in a nanorod.
We form a single junction between p-type Cu2S and n-type CdS through a controlled
cationic exchange process of CdS nanorods. We characterize nanorods of the individual
materials and the single junction in a nanorod with an ultrahigh vacuum scanning
tunneling microscope (UHV-STM) at 77 K. From scanning tunneling spectroscopy and
correspondingly the density of states (DOS) spectra, we determine the conduction and
valence band-edges at different points across the junction and the individual nanorods.
We could map the band-diagram of nanorod-junctions to bring out the salient features of
a diode, such as p- and n-sections, band-bending, depletion region, albeit interestingly in
the nanoscale.
KEYWORDS: pn-junction in a nanorod, scanning tunneling spectroscopy, mapping of band-edges across a pn-junction in a nanorod,
density of states
B
the surface, the ligands would perturb the interface between the
two materials or the junction itself. We hence have targeted to
form a pn-junction in a single nanorod instead. To do so, we
have formed a junction between CdS and Cu2S in the nanorod
through a cationic exchange process that acted as n- and p-type
materials, respectively. We mapped the bands of the junction in
a nanorod with an aim to visualize the electric field across the
depletion region of the diode formed in a single nanorod.
Characterization of Nanorods and Junctions: Optical
Absorption, Transmission Electron Microscopy (TEM),
High-Resolution TEM (HR-TEM) Images, and STM Topography. We have recorded optical absorption spectra of CdS
and Cu2S nanorods and also of the CdS|Cu2S junctions (Figure
1). In characterizing nanorod-junctions, we chose two reaction
times for the cationic exchange process that in effect controlled
the length of Cu2S in the CdS|Cu2S junction. While the
spectrum of CdS shows absorption in the short wavelength
region, the one of Cu2S extends also to the near-IR region. The
absorbance in the 475−650 nm region appeared solely due to
Cu2S; the long wavelength region of the spectrum arises due to
the localized surface plasmon resonance occurring out of
copper deficiencies.27 Both the spectra matched the reported
results of nanorods (and of other lower dimensional structures)
of the respective materials.27−29 The spectra of the CdS|Cu2S
junctions were mostly the sum of individual ones with the
absorbance in the 475−650 nm regions increasing with the
and-edges of lower dimensional structures have been one
of the major parameters of importance in designing an
electronic device. Accordingly, shrinking the length of active
materials along one to three directions leading to quantum
confinement effect,1−3 doping,4 and alloying5 of the semiconductors, and so forth have been considered as routes to alter
the band-edges in one way or the other. Junctions between two
semiconductors in the form of type-I,6,7 type-II or staggered,8,9
and type-III or type-II broken gap10 band-offset are inherently
interesting systems so far as the energy-levels at the interfaces
are concerned. Such band-offsets are formed in core−shell
nanoparticles11−14 and nanorods/nanowires15 and in conventional pn-junctions in a nanorod.16,17 The offsets at the
interface of a heterostructure, along with the location of bandedges, are hence important in engineering of nanostructures.
In locating the band-edges of lower-dimensional systems,
scanning tunneling spectroscopy (STS) employing a scanning
tunneling microscope (STM) has so far been the most
powerful technique.4,14,18−22 The ability to map band-edges
of nanostructures has made the technique unique in many
respects. The nanostructures in this direction ranged from
simple quantum dots4,19 to complex core−shells14,18,21 and
hybrid core−shell systems in which organic molecules formed
the shell layer on inorganic nanocrystals.20,22 In research driven
by applications, cross-sectional STM23−25 has been used to look
at the interface between the materials of bulk-heterojunction
solar cells.24
In this work, we aimed to map the bands in a pn-junction
nanostructure. In a junction between two separately formed
nanostructures, because they are grown through colloidal
synthesis routes17,26 involving organic ligands as stabilizers at
© 2014 American Chemical Society
Received: January 8, 2014
Revised: February 15, 2014
Published: March 3, 2014
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Figure 1. Optical absorption spectra of CdS and Cu2S nanorods and
of CdS|Cu2S junctions. In the nanorod-junctions, CdS|Cu2S (1) and
CdS|Cu2S (2) represent junctions formed with a reaction time of 7
and 10 min, respectively, for the cationic exchange process that in
effect controlled the length of Cu2S section in the nanorod-junctions.
Figure 3. Scanning tunneling microscopy (STM) topography of (a)
CdS and (b) Cu2S nanorods and (c) Cu2S|CdS and (d) Cu2S|CdS|
Cu2S nanorod junctions. Set-points for the approach of the tip were
0.4 nA at 2.0 V. These set-points were used during STS measurements.
appeared brighter than the CdS presumably due to a higher
conductivity of Cu2S that would have led to an increased tip-tonanorod distance as compared to CdS. From the difference in
brightness and also wideness, we could identify the materials in
a nanorod for further characterization under STM. The
diameter of CdS and Cu2S nanorods or their sections in a
junction were both around 5 nm. With the exciton Bohr radius
in CdS and Cu2S being 3.0 nm and in the range of 3−5 nm,
respectively, both the materials in nanorods that we
characterized were close to their bulk form.30,31
In the CdS and Cu2S nanorods, while probing different
points across the nanorods with a STM tip, our aim was to
locate the conduction and valence band-edges at different
points on the nanorods. To do so, we recorded tunneling
current versus voltage (I−V) characteristics with the STM tip
as the electrode for tunneling of charge carriers. Here, the
tunneling current was recorded after positioning the STM tip
above the nanostructure and disabling the scanning and
feedback controls. Because the system resembled a doublebarrier tunnel junction (DBTJ) involving the tip−nanorod and
nanorod−substrate tunnel barriers,32 we carried out the
measurements at different tip−nanorod distances by using
different set-points for the approach of the tip. We then
calculated the density of states (DOS) so that the band-edges
could be located. It may be stated here that because bias was
applied with respect to the tip, the peaks at the positive voltage
region, at which electrons can be injected from the tip to the
nanorod, denote the location of conduction bands. Similarly,
increase in the length of Cu2S section in CdS|Cu2S nanorod
junctions.
High-resolution TEM (HR-TEM) images of CdS and Cu2S
nanorods separately, as presented in Figure 2a,b, respectively,
show that the length of the nanorods was about 70 nm in both
the cases. In the nanorod junctions, the dark-field scanning
transmission electron microscopy (STEM) image, as shown in
Figure 2c, confirms the presence of two phases in a nanorod. In
some nanorods, Cu2S appeared to have formed from both ends
of CdS. The images favorably implied that the Cu2S section of
the nanorod-junction was bright and wider. The HR-TEM
image further shows that though the nanorods were largely
single crystalline in nature, two clear crystalline planes are
visible in a single nanorod evidencing formation of a junction in
a nanorod. An interlayer spacing of 0.338 and 0.340 nm at the
two parts of a nanorod matched well with the ⟨002⟩ plane of
CdS nanocrystals (hexagonal phase) and the ⟨002⟩ plane of
Cu2S nanocrystals (chalcocite-type hexagonal phase), respectively. The coparison has been carried out with JCPDS files 840208 and 80-0006, respectively).
STM Topography. We have recorded STM topography of
CdS and Cu2S nanorods and also of the CdS|Cu2S junctions
(Figure 3). Some of the junctions included Cu2S|CdS|Cu2S
nanorods where Cu2S formed from both ends of CdS. While
the topographies of CdS and Cu2S were uniform, the ones for
the junctions exhibited a difference in topographies in the CdS
and Cu2S sections of nanorod junctions. In imaging the
junctions in a constant-current mode, the Cu2S section(s)
Figure 2. (a) A TEM image of CdS nanorods, (b) that of Cu2S nanorods, (c) a dark-field TEM image of Cu2S|CdS and Cu2S|CdS|Cu2S junctions,
and (d) HR-TEM image of a Cu2S|CdS junction showing interlayer spacing of different sections.
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the peaks at negative voltages denote the valence band at which
electrons can be withdrawn from the nanorods. The energies
shown in the DOS spectra are with respect to the Fermi energy
of the material.
Tunneling Current and Density of States (DOS). The
I−V characteristics at five different points on CdS and Cu2S
nanorods are shown in Figures 4 and 5, respectively. STM
Figure 5. (a) STM topography of a Cu2S nanorod showing the spots
at which (b) tunneling current versus voltage characteristics were
recorded. (c) DOS spectra of the I−V characteristics at the spots on
the nanorod. The broken lines indicate the location of conduction and
valence band-edges.
tip was very close to the nanorods, the other tunnel-barrier of
the DBTJ started to influence the results.
With the positive and negative voltages at which peaks
appeared in DOS spectra denoting the location of conduction
and valence bands, respectively, we find that the conduction
band in CdS is located closer to the Fermi energy that is fixed
at 0 V. Similarly, for the Cu2S case the valence band is closer to
its Fermi energy. The DOS spectra are hence in agreement with
the n- and p-type nature of CdS and Cu2S nanorods,
respectively. The position of the band-edges and also the
intensity of DOS spectrum at different points on a nanorod did
not vary when we scanned across a nanorod. The bandgap of
CdS and Cu2S turned out to be 2.8 and 1.3 eV, respectively,
that match reasonably well with the literature. The actual
bandgap would be a little lower if the voltage drop at the
nanorod−substrate junction of DBTJ was considered. From the
DOS spectra of CdS and Cu2S nanorods, we find that the
bandgap of Cu2S was clearly lower than that of CdS. The
materials moreover would form a type-II band-alignment if a
junction is formed between them.
CdS|Cu2S Single Junction in a Nanorod. We then
proceeded to characterize CdS|Cu2S nanorod junctions. To
identify a nanorod with a single junction instead of Cu2S|CdS|
Cu2S junctions, we relied on STM topographies, a typical of
which is shown in Figure 6. We recorded I−V characteristics on
such a single junction nanorod at many different points. We
Figure 4. (a) STM topography of a CdS nanorod showing the spots at
which (b) tunneling current versus voltage characteristics were
recorded. (c) DOS spectra of the I−V characteristics at the spots on
the nanorod. The broken lines indicate the location of conduction and
valence band-edges.
topography of the two nanorods with spots at which I−V
characteristics were recorded is shown in the respective figures.
The asymmetry in the I−V characteristics have arisen due to
the difference in the work-function of the two electrodes. For
CdS nanorods, the characteristics yielded a higher current in
the positive voltage region at which electrons are injected to the
conduction band of the n-type material, as compared to the
current in the negative voltage section. Similarly, for the p-type
Cu2S, the current remained higher at the negative voltage
region due to facile hole injection to (electron extraction from)
the valence band. The I−V characteristics expectedly did not
vary from a point to another, because a single material was
being characterized across the nanorod. The DOS spectra of
the nanorods evidenced the location of conduction and valence
band-edges in both the cases. The position of the band-edges
did not depend on the set-points used for the approach of the
tip (Figures S1a,b in the Supporting Information). When the
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Figure 7. Mapping of a CdS|Cu2S nanorod junction for conduction
and valence band-edges along the length of the nanorod. The figure in
addition shows the STM topography of the nanorod junction and the
spots at which tunneling current was measured. Conduction and
valence band-edges (CB and VB, respectively) along with the Fermi
energy (EF) across the junction and the depletion region of the pnjunction are also shown in the figure.
nanoscale. The results enabled us to visualize the bandpositions along the interface between the CdS and Cu2S
sections in a junction. The figure shows that in a 70-odd
nanometer nanorod junction, the depletion region extends to
around 42 nm.
Schematic Band-Diagram of a pn-Junction in a
Nanorod. Finally, we present schematic band-diagrams of
Cu2S and CdS before and after formation of a pn-junction
(Figure 8). While the “before-contact” diagram was based on
Figure 6. (a) STM topography of a CdS|Cu2S nanorod junction
showing the spots at which (b) tunneling current versus voltage
characteristics were recorded. (c) DOS spectra of the I−V characteristics at the spots on the nanorod. The broken lines indicate the
location of conduction and valence band-edges.
focused at the interface between CdS and Cu2S sections of the
nanorod by characterizing points at shorter interval of distance.
Some such I−V characteristics are shown in Figure 6. The
corresponding DOS are also shown in the figure. We looked at
the characteristics and the DOS spectra for conduction and
valence band-edges. We noted that while the ends of the
nanorods resembled the individual materials, the interface
brings out interesting features. In Figure 7, we have plotted the
band-edges across the whole nanorod junction. The points at
which measurements were carried out have also been marked in
the STM topography of the CdS|Cu2S nanorod junction. On
each point on the nanorod, we have recorded at least 50 I−V
characteristics in determining the conduction and valence bandedges from the DOS spectra. The 50 measurements at each
point have been summed up as error bars in energy values in
Figure 7. The figure in essence is a mapping of the conduction
and valence band-edges across a single pn-junction in a
nanorod. The band-edges at the terminals resembled that of the
individual nanorods, that is, CdS and Cu2S, as shown in Figures
4 and 5, respectively. The band-edges at the interface bring out
the band-bending along with the depletion region of a pnjunction diode that formed in a single nanorod having a length
of about 70 nm. In other words, we could map the conduction
and valence bands along a single pn-junction formed in a
Figure 8. Schematic band-diagrams of Cu2S and CdS nanorods before
and after formation of a pn-junction.
DOS of individual nanorods (Figures 5 and 6), the banddiagram of “after-contact” pn-junction was plotted from the
band-mapping along the nanorod-junction. A type-II bandalignment of the materials along with the p- and the n-nature of
Cu2S and CdS nanorods, respectively, were instrumental in
forming a pn-junction in a nanorod. The position of band-edges
in individual nanorods falls in the range of reported results for
lower-dimensional structures of the respective materials. The
band-edges at the extreme ends of the junction also matched
that of individual components. The energy levels at the
interface of the pn-junction matched well with a conventional
diode junctions having a band-bending of around 0.3−0.4 eV.
The results hence show the elegance of STM in determining
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1 mL) was added dropwise to the flask through a syringe, which
contained 1.13 mg of CdS nanorods in toluene under a
vigorous stirring condition at room temperature. The process of
exchange of cadmium by copper ions from both ends of the
CdS nanorods leading to the formation of Cu2S|CdS|Cu2S
nanorods is manifested by an immediate change in color of the
reaction bath from yellow to golden brown. The conversion
process could further be slowed down for partial exchange of
cadmium from one of the ends of CdS nanorods by a 5-fold
dilution of the copper salt. In such a case, CdS|Cu2S junctions
formed in the nanorods. The nanorods were washed with an
addition of methanol followed by centrifugation and removal of
the supernatant.
To form Cu2S nanorod, we added a required amount of
[Cu(CH3CN)4]PF6 so that all the cadmium atoms were
replaced by copper. In practice, a methanol solution of the
copper salt (4.8 mg in 0.7 mL) was added dropwise to a
dispersed solution of CdS nanorods in toluene (0.113 mg in 2.0
mL).
Characterization of CdS and Cu2S Nanorods and CdS|
Cu2S Junctions. Optical absorption spectroscopy and TEM and
HR-TEM images of CdS and Cu2S nanorods and CdS|Cu2S
junctions in nanorods were recorded. Bright-field TEM images
were obtained using a JEOL transmission electron microscope
that was operated at 200 kV. For TEM measurements, a drop
of nanorod solution was placed onto a carbon-coated copper
grid in an ambient condition. Statistics for the length and
diameter of the initial CdS nanorods and fully converted Cu2S
nanorods were gathered from bright-field TEM images using
Gatan software. At least 20 measurements were made for each
sample.
To characterize individual nanorods and CdS|Cu2S junctions
through an ultrahigh vacuum STM (PAN style UHV-STM of
M/s RHK Technology), ultradilute solution of the nanorods
was drop-casted on highly ordered pyrolytic graphite (HOPG).
While the pressure of the microscope was 1.2 × 10−10 Torr, the
temperature of the substrate and the Pt/Ir tip both was 77 K.
During approach of the tip, a current of 0.4 nA was targeted to
achieve at 2.0 V. For smoothing of STM topographies, WSxM
software was used. As per usual procedure,31 we used a low tipapproaching current to ensure that the bias was applied mainly
on the tip-nanorod junction.
and more importantly mapping the band-edges across a pnjunction in a single nanorod.
In conclusion, we have grown a CdS|Cu2S junction in a
nanorod through a cationic exchange process. Formation of
such a junction in a single nanorod has been substantiated by
TEM, HR-TEM, dark-field TEM images, and STM topographies. With UHV-STM, we have mapped the nanorodjunction and also individual nanorods across their length by
recording tunneling current at different points on the nanorods
at 77 K. From the DOS spectra, we could locate the conduction
and valence band-edges across the Cu2S and the CdS nanorods
and also along the pn-junction. The band-edges at the terminals
of a pn-junction resembled that of CdS and of Cu2S nanorods,
respectively. The DOS spectra across the pn-junction yielded a
band-bending along with the depletion region in the junction.
The results showed a unique method to map the conduction
and valence band-edges along a single pn-junction nanorod
based on CdS and Cu2S.
Experimental Section. Growth of CdS Nanorods. CdS
nanorods were grown following a colloidal synthetic method
using a standard Schlenk line technique26 in an inert condition.
In brief, CdO and measured amounts of ODPA (octadecylphosphonic acid), TDPA (tetradecylphosphonic acid), and
TOPO (trioctylphosphine oxide) were added in a 25 mL threeneck flask. The flask was purged with nitrogen and heated to
120 °C that melted the components to remove water from the
materials. The mixed content was heated to 320 °C for 20−30
min to enable the complexation of cadmium ions. The process
of heating to 320 °C was repeated once more to complete the
complexation process. At this stage, 2 g of TOP (trioctylphosphine) was injected into the flask that acted as the stabilizer.
After stirring the liquid at 320 °C for 5 min, TOPS
(trioctylphosphine sulfide), as a source of sulfur, was injected
to the flask. Reaction was allowed to continue for 60 min to
form CdS nanorods. Reaction was stopped by reducing the
temperature of the flask down to room temperature.
Washing of the nanorods was carried out by adding three
solvents in sequence, namely toluene, toluene/octylamine (1:1
v/v), and toluene/acetone (1:1 v/v), followed by centrifugation
after each step. Washing in toluene/acetone more importantly
allowed separation of bipods, tripods, and tetrapods from the
nanorods. Here, after centrifugation, the branched structures
remained in the supernatant.
Formation of a CdS|Cu2S Junction in Nanorods through a
Cationic Exchange Process. We formed a junction between
CdS and Cu2S in the nanorod following a reported route.26
Here copper-ions replaced the cadmium ions from one or both
ends of the nanorods. To do so, tetrakis(acetonitrile)copper(I)
hexafluorophosphate, [Cu(CH3CN)4]PF6 acted as the source
of copper. Such a salt was used in the cationic exchange process
due to a slow reactivity of copper in the salt so that the
exchange of cadmium by copper ions remained an extremely
slow process. Moreover, the replacement of cadmium occurred
only at the end(s) of the nanorod since the surfactants are
known to be weekly bound to the edges of a nanorod.33 In
practice, while CdS nanorods were dispersed in toluene,
[Cu(CH3CN)4]PF6 was dissolved in methanol. Concentration
of copper ions was chosen to match the molar concentration of
cadmium ions to be replaced in the nanorods. The
concentration of cadmium ions in CdS nanorods was
determined from inductively coupled plasma atomic emission
spectroscopy (ICP-AES) Because toluene and methanol are
miscible, a methanol solution of [Cu(CH3CN)4]PF6 (4.8 mg in
■
ASSOCIATED CONTENT
S Supporting Information
*
DOS spectra calculated from I−V characteristics of CdS
nanorod and Cu2S nanorod recorded at different set-points
with a set-bias of 2.0 V. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Tel: +91-33-24734971. Fax: +9133-24732805.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors acknowledge financial support through Nano
Mission projects. A.B. and S.D. acknowledge CSIR Fellowship
Nos. 09/080(0779)/2011-EMR-I (Roll No. 510847) and 9/
080(0647)/2009-EMR-I (Roll No. 507031), respectively.
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