Rectifying Junctions from an Assembly of Two Dissimilar

Feb 12, 2008 - We report current rectification in a junction between two different ... Sanjini U. Nanayakkara , Gilad Cohen , Chun-Sheng Jiang , Manue...
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J. Phys. Chem. C 2008, 112, 3232-3238

Rectifying Junctions from an Assembly of Two Dissimilar Nanoparticles Kallol Mohanta and Amlan J. Pal* Department of Solid State Physics and Centre for AdVanced Materials, Indian Association for the CultiVation of Science, Kolkata 700032, India ReceiVed: August 17, 2007; In Final Form: December 5, 2007

We report current rectification in a junction between two different nanoparticles. A monolayer of p-type nanoparticles (PbSe or PbS) and a monolayer of n-type nanoparticles (CdSe or CdS) assembled in sequence form such a junction. Here the nanoparticles, which have been characterized following standard experimental methods, are suitably capped to facilitate electrostatic assembly. While keeping the sequence of capping materials the same, if the order of the monolayers is reversed, the direction of current rectification becomes opposite. Junctions between two p-type or two n-type nanoparticle monolayers, on the other hand, do not exhibit current rectification. The rectifying and inverse-rectifying characteristics from pn and np junctions, respectively, and nonrectifying pp and nn junctions rule out any role of interfaces in observing the rectification. The results presented in this article show that rectifying nanodiodes are achieved from a suitable assembly of two nanoparticles.

1. Introduction

2. Experimental Section

With the advent of nanoparticles, research on electronic and optoelectronic devices has changed its dimension.1-6 Semiconducting nanoparticles are forerunners in this direction due to their ability to tune band gap with physical parameters like size and shape.7-9 Depending on the nature of devices, low- or highband gap semiconductors are being used. In this direction, while PbS and PbSe fall in the class of low-band gap semiconductors, CdS and CdSe belong to the other category. Assembly of two categories of nanoparticles in a rational way can further enhance the novelty of the properties.2,5,10-13 Two different nanoparticles can be assembled by use of suitable capping agents or stabilizers.2,5 With suitable functional groups, adsorption of the particles on the substrates14 or adhesion between two different nanoparticles can be achieved.2,5 Adsorption of the nanoparticles occurs via layer-by-layer (LbL) electrostatic absorption process through anionic or cationic capping agents.1-5,14-17 By sequencing the nature of capping materials, a monolayer of one type of nanoparticle can hence be adsorbed on top of a monolayer of the other one to form a junction between two different types of nanoparticles. If there is a variation in the nature of free carriers in the two particles, such a junction between them may exhibit electrical rectification or (nano)diode characteristics.5 Nanodiodes or junctions between two nanoparticles have been achieved by controlling doping levels of the individual components. Use of dissimilar semiconductors with a large variation in band gap may offer many additional options in fabricating a suitable junction. In this paper, we show processes to form junctions between PbSe or PbS and CdSe or CdS nanoparticles. Here the Pb-based semiconductors have a much lower band gap than the Cd-based ones. Moreover, the nature of free carriers in the two systems also differs. In this paper, we present characteristics of junctions formed between different kinds of nanoparticles.

Growth of Nanoparticles. PbSe, PbS, CdSe, and CdS nanoparticles were grown following standard protocol.18 While lead acetate and cadmium acetate were used as Pb and Cd ion sources, respectively, sodium selenosulfate (Na2SeSO3) and sodium sulfide (Na2S) served as the sources of Se and S ions, respectively. Growth of the nanoparticles was controlled by use of mercaptoacetic acid (MAA) as a capping agent. To grow CdSe nanoparticles, 40 mL of 2 mM cadmium acetate and 40 mL of 2 mM MAA were mixed together and stirred vigorously. The pH of the mixed solution was set to 12 by adding NaOH. Freshly prepared 20 mM Na2SeSO3 (2 mL) was then added during continuous stirring. The mixed solution gradually turned yellow, indicating formation of CdSe nanoparticles. To form CdS nanoparticles, the pH of cadmium acetate/MAA mixed solution was set to 10.5 before addition of S ion source (Na2S). In this case, the mixed solution turned greenish-yellow. Similarly, with lead acetate, PbSe and PbS nanoparticles were formed. The pH of lead acetate/MAA mixed solution was set to 8.0 and 7.0 for the growth of PbSe and PbS nanoparticles, respectively. With the addition of Se or S ion source, the solution turned brownish-black or silky chocolate black, respectively. After completion of the reaction, the solutions were centrifuged at 18 °C at a speed of 14 000 rpm. The precipitates were collected and further dissolved in deionized water (Milli-Q Academic System; resistivity ) 18.2 MΩ‚cm). The washing processes were repeated several times to isolate MAA-capped nanoparticles. To introduce cationic capping, poly(diallyldimethylammonium chloride) (PDDA) was added to MAA-capped nanoparticles. After 30 min of vigorous stirring, the solution was centrifuged to remove excess PDDA. The procedure was repeated several times after the nanoparticles were redissolved in water. Characterization of Nanoparticles. Composition of the nanoparticles was estimated by energy-dispersive X-ray (EDX) analysis. The crystalline phase of the nanoparticles was studied by X-ray diffraction patterns (Rich-Seifert XRD 3000P) and high-resolution transmission electron microscope (HR-TEM)

* Corresponding author: e-mail [email protected].

10.1021/jp076624+ CCC: $40.75 © 2008 American Chemical Society Published on Web 02/12/2008

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Figure 1. XRD spectra of CdSe, CdS, PbSe, and PbS nanoparticles along with their EDX analysis and HR-TEM images.

images (JEOL JSM 6700F). Size of the nanoparticles was estimated from the bandwidth of XRD spectra, HR-TEM, and electronic absorption spectra (Shimadzu UV-2550 UV-visible spectrophotometer). Signatures of the capping agents were verified from Fourier transform infrared (FT-IR) spectra. Monolayer Formation. Thin films of the nanoparticles were formed via a layer-by-layer (LbL) electrostatic adsorption process. Films were deposited on n-type Si(111) wafers (As doped; resistivity ) 3-10 mΩ‚cm). The substrates were deprotonated with a 5:1:1 H2O/NH4OH/H2O2 solution to facilitate adsorption of a cationic layer. Deposition of a monolayer of PDDA-capped nanoparticles was carried out by dipping the substrate in a dispersed solution for 15 min following rinsing in three deionized water baths. Formation of a monolayer of the nanoparticles was verified by measuring the depth of a scratched film through atomic force microscopy (AFM) measurements. Growth of a monolayer during the LbL deposition process was substantiated by repeating the adsorption sequence. That is, an electronic adsorption spectrum was recorded after deposition of each layer of (PDDA-capped nanoparticle/PAA)5, (MAA-capped nanoparticle/PAH)5, or (PDDA-capped nanoparticle/MAA-capped nanoparticle)5 LbL film. Here polyacrylic acid (Mw ) 240 000) and poly(allylaminehydrochloride) (Mw ) 70 000), abbreviated as PAA and PAH, respectively, were used as a polyanion and a polycation, respectively, during LbL deposition. Junction Formation. To form a junction between two monolayers of two different types of nanoparticles, a monolayer of one type was deposited on top of a monolayer of the other one. As necessitated by the LbL deposition process, which relies on electrostatic absorption process due to surface charge reversal, we chose different nature of the cappings for the particles in the two monolayers. For example, a junction between CdSe and PbSe was formed by depositing a MAA-capped PbSe monolayer on top of a PDDA-capped CdSe monolayer. Reverse

junction (PbSe/CdSe) was formed by depositing a PDDAcapped PbSe monolayer and a MAA-capped CdSe monolayer in sequence. Similarly, all possible junctions between CdSe, CdS, PbSe, and PbS monolayers were formed. Sequence of the capping agents was kept the same in all the junctions; that is, a monolayer of PDDA-capped nanoparticles was always deposited on Si wafers; deposition of a monolayer of MAA-capped nanoparticles completed formation of a junction. In addition, as control experiments, individual monolayers were also deposited and characterized. A junction between two monolayers of the same material but with different capping agents (for example, PDDA-capped CdSe/MAA-capped CdSe) was also characterized. Characterization of Junctions. Electrical characteristics of different junctions and their components were characterized with a Pt/Ir tip of a scanning tunneling microscope (STM, Nanosurf easyScan2) controller in noncontact (tunneling) mode under ambient conditions. Here, the tip was approached till a preset current value (at a particular voltage) was achieved. A topographic image of the surface was recorded before currentvoltage (I-V) characteristics were measured at different points on the film. Voltage was scanned toward both bias directions and at different sweep speeds. 3. Results and Discussion Characterization of Nanoparticles. Figure 1 shows XRD patterns of CdSe, CdS, PbSe, and PbS nanoparticles. Patterns of all the particles show distinct peaks corresponding to the crystal planes of the materials with suitable sharpness. The CdSe nanoparticles show peaks due to 100, 002, 101, 110, and 103 planes (compared with diffraction data, JCPDF file 080459). Similarly, the diffractogram of the CdS nanoparticles exhibits peaks due to 111, 200, 220, 311, and 331 planes (diffraction data, JCPDF file 100454). The peak positions of PbSe and PbS

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Figure 2. Plots of (β cos θ)/λ vs (sin θ)/λ for CdSe, CdS, PbSe, and PbS nanoparticles. Size of the nanoparticles, derived from Hall equation, is shown in parentheses in the legend.

nanoparticles, as depicted in Figure 1, have matched accordingly with diffraction data, JCPDF files 060354 and 050592, respectively. Composition of the nanoparticles, as examined by EDX analysis, provided equivalent atomic ratio of the components (Figure 1, inset). HR-TEM images of the particles show the lattice spacing corresponding to the crystalline structure of the materials (Figure 1). Both the results and the XRD patterns confirm chemical composition of the nanoparticles in their crystalline phase. Particle size can also be estimated from the images. The size can also be calculated from the broadening of XRD peaks (as shown in Figure 1). The calculation is based on the Hall equation:19

β cos θ 1 sin θ ) +η λ  λ where β is the fwhm (full width at half-maximum ≡ broadening of peak) measured in radians, λ is the irradiation wavelength (1.5405 Å),  is the effective particle size, and η is the effective strain. Plots of (β cos θ)/λ versus (sin θ)/λ for the four particles are shown in Figure 2. The y-intercept provides the inverse of effective particle size. From the plots, particle sizes for CdSe, CdS, PbSe, and PbS nanoparticles have been estimated as 8.0, 14.0, 12.3, and 7.9 nm, respectively. We have recorded UV-visible absorption spectra of CdSe and CdS nanoparticles in dispersed solution. The PbSe and PbS nanoparticles, which have a much lower band gap, expectedly did not show any absorption band in the 200-800 nm range of our measurement. Absorption band for CdSe and CdS nanoparticles appeared at 499 and 459 nm, respectively. Size of the nanoparticles has been calculated also from the absorption spectra by use of the effective mass equation:20

h R) Egn - Egb

x[ ] Egb 2m*

where R is the particle size, h is Planck’s constant, m* is the effective mass of an electron, Egb is the band gap for bulk, and Egn is the band gap of the nanoparticles. The estimated sizes of CdSe and CdS nanoparticles are around 8.1 and 16.5 nm, respectively, which match reasonably well with those obtained from the calculation based on the broadening of XRD patterns. Junction Formation. To form a junction between two different nanoparticles, we have deposited a monolayer of one type of nanoparticles on top of a monolayer of another type.

Figure 3. UV-visible absorption spectra of LbL film by depositing (a) MAA-capped CdS nanoparticles with PAH and (b) PDDA-capped CdSe nanoparticles with PAA, in repeated cycles. (c) Spectra during deposition of PDDA-capped PbSe and MAA-capped CdS layers. The arrows in each of the figures point to the direction of increase in the number of layers. (Insets) Absorbance (at peak wavelength) vs number of LbL layers deposited.

Adsorption of nanoparticles via LbL technique has been verified by depositing MAA-capped particles with a polycation (PAH) or PDDA-capped nanoparticles with a polyanion (PAA) in repeated cycles. UV-visible absorption spectra recorded after adsorption of each bilayer confirmed deposition of the nanoparticles. Figure 3 shows the spectra of LbL films of (a) CdS nanoparticles with PAA polyions and (b) CdSe nanoparticles with PAH polyions. Insets show plots of peak absorbance versus number of layers deposited in the LbL process. In each case, a linear plot with a slope of unity confirms that the nanoparticles were adsorbed uniformly in each layer. Adsorption of two monolayers of two different nanoparticles in sequence has similarly been verified by recording UV-visible absorption

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Figure 4. (a) Film thickness versus layer number of CdSe LbL films. PDDA-aand MAA-capped CdSe were used as cationic and anionic solution for deposition of LbL films. A typical AFM image (5 µm × 5 µm) of a scratched film and the depth profile of the scratch are shown in panels b and c, respectively. (c) Topographic cross section over a segment of 1.85 µm; the vertical scale is 35.9 nm.

spectra after deposition of each monolayer. That is, we have formed five bilayers of LbL films of two differently capped nanoparticles and have recorded the spectrum after deposition of each monolayer. In Figure 3c, we show UV-visible absorption spectra of PbSe/CdS LbL film as it was built up layer by layer. Here, PbSe and CdS were capped with PDDA and MAA, respectively. The absorption intensity corresponding to CdS nanoparticles increases linearly with the number of layers (Figure 3c, inset). The PbSe layers, which do not absorb in the spectral range of measurement, provide only a small rise in the background. The results hence show that the CdS nanoparticles were adsorbed uniformly during PbSe/CdS LbL deposition. The results further confirm deposition of PbSe layers, because without adsorption of a cationic PbSe layer, a CdS monolayer (which is anionic in nature) would not have adsorbed subsequently in the LbL deposition process. To confirm that a monolayer of the nanoparticles was indeed deposited during a dipping, we recorded AFM images after deposition of each layer of LbL film. Before the images were recorded, we made a single scratch on the film with a clean and sharp object. The depth of the scratch (or well) was measured for all the films. The depth versus layer number plot is linear with a slope of unity (Figure 4a). Depth for a monolayer matched with the diameter of the nanoparticle. This confirms that only a monolayer of the nanoparticles was deposited during the electrostatic absorption process. Figure 4b shows typical AFM topography of a scratched monolayer, while Figure 4c shows the depth profile displaying the measurement technique.

The AFM image also shows the homogeneity of the surface of the LbL films of nanoparticles. Current-Voltage Characteristics of Junctions. We have recorded I-V characteristics of different junctions (based on assembly of two monolayers of two different nanoparticles) with an STM tip. Characteristics of junctions based on a high and a low band gap material (e.g., PbSe/CdS, PbSe/CdSe, PbS/CdS, and PbS/CdSe) are presented in Figure 5. The plots exhibit rectification, with current flow being favorable from Pb-based particles to Cd-based ones. The rectification ratio, which depends on the voltage, reaches up to 35. Since the former are intrinsically p-type and the latter ones are n-type in nature, such rectification points to favorable current flow from a p-type to an n-type nanoparticle. To validate the rectifying properties of a junction between two nanoparticles, we characterized reverse junctions. In each of the plots of Figure 5, characteristics from their reverse junctions (e.g., CdS/PbSe, CdSe/PbSe, CdS/PbS, and CdSe/PbS, respectively) are also presented. The plots are mirror image to those of the forward junction, showing rectification in the opposite direction. In other words, independent of the sequence of particles in the junction, current flow is always favorable from a p- to an n-type nanoparticle. Rectification ratio (calculated at a particular voltage) of a junction matches that of its reverse junction. For each junction between a pair of nanoparticles, I-V characteristics under multiple loops were invariant, showing no dependence on voltage-sweep directions. Characteristics at different points on the bilayer film or with different maximum

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Figure 5. I-V characteristics of junctions based on a monolayer of Pb-based nanoparticles (PbSe or PbS) and a monolayer of Cd-based nanoparticles (CdSe or CdS). Characteristics of each junction have been compared with its inverse one.

Figure 6. I-V characteristics from control experiments: junctions between (a) a monolayer of PDDA-capped CdSe nanoparticles and a monolayer of MAA-capped CdSe nanoparticles and (b) a monolayer of PbS, PbSe, CdS, and CdSe nanoparticles. In panel a, to deposit a MAA-capped CdSe monolayer on Si wafers, the substrates were treated with 2-propanol/trichloromethoxysilane (10:1).

voltage (Vmax) were reproducible. We have calculated average ideality factor of these junctions using Shockley’s diode equation:

eV - 1) (NkT

I ) I0 exp

where I0 is the reverse saturation current, e is the electronic charge, k is the Boltzmann constant, T is the absolute temperature, and N is the diode ideality factor. From one junction to the other, the average ideality factor varies between 1 and 2.21-23 This suggests that current flow is due to thermionic emission. Control Experiments. The results presented in Figure 5 inherently rule out the effects of interfaces and capping agents in the observed current rectification. In the junction, PDDA-

capped particles were always deposited on Si wafers and MAAcapped ones as the top monolayer. When a junction is compared with its reverse one (e.g., PbSe/CdS with CdS/PbSe), the interfaces with the electrodes are the same in the two cases. That is, PDDA-capped particles are always in contact with Si while MAA-capped ones are in contact with Pt/Ir (in noncontact mode). For example, Figure 5a shows that current rectification in a junction between PDDA-capped PbSe and MAA-capped CdS monolayers in sequence is opposite to that in a junction of PDDA-capped CdS and MAA-capped PbSe monolayers in sequence. All the plots in Figure 5 show that, though the sequence of interfaces with the electrodes and the interface between two nanoparticles remain the same, the direction of current rectification depends solely on the sequence of the two

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Figure 7. I-V characteristics of (a) nn junctions between a monolayer of CdSe nanoparticles and a monolayer of CdS nanoparticles and (b) pp junctions formed by two p-type nanoparticles, namely, a monolayer of PbSe nanoparticles and a monolayer of PbS nanoparticles.

nanoparticles. In other words, the interfaces with the electrodes and the interface between two capping agents do not yield the observed rectification. Further control experiments have been performed by characterizing a junction between two monolayers of the same nanoparticles with different capping materials. Such a junction (for example, a PDDA-capped CdSe monolayer and a MAAcapped CdSe monolayer in sequence), and its reverse one, yielded symmetric I-V characteristics (Figure 6a). The results hence again rule out the role of the interfaces. Components of the junctionssa monolayer of each of the four nanoparticless have been characterized under identical experimental condition. I-V characteristics of the nanoparticles are in general symmetric in nature (Figure 6b). Such a behavior is true for sweeps in both directions and also over many cycles. A little asymmetry in Pb-based particles, which are p-type in nature, could be due to the film (junction) formation on n-type Si wafers. The rectification ratio in PbSe and PbS monolayers was, however, much lower as compared to that with junctions between monolayers of Pb- and Cd-based semiconducting nanoparticles. The results from the control experiments hence confirm that the observed electrical rectification is indeed due to a junction between two different nanoparticles. Mechanism. The observed rectification from a junction between a monolayer of p-type and a monolayer of n-type nanoparticles prompted us to analyze the results in terms of a pn junction, where formation of a depletion layer results in barriers and subsequently unidirectional flow of current. To verify that a junction between a p- and an n-type nanoparticle is indeed required to observe current rectification, we chose a pp and an nn junction. That is, we characterized junctions between a monolayer of CdSe and a monolayer of CdS nanoparticles in both sequences. Similarly, we characterized PbSe/PbS and PbS/PbSe junctions formed by the assembly of two monolayers. Here also, I-V characteristics were symmetric (Figure 7) for voltage sweeps in both directions. The results show the requirement for a p- and an n-type nanoparticle in forming a rectifying junction. It should be noted that, in these nonrectifying junctions, the interfaces with the electrodes and the interface between the nanoparticles are the same as the rectifying junctions (Figure 5). Absence of rectification in these pp and nn junctions (Figure 7) hence further confirms that the interfaces do not yield any rectification. The results therefore show that rectifying nanodiodes can be achieved only from a combination of p- and n-type nanoparticles.

4. Conclusion In conclusion, we have demonstrated current rectification in a junction between monolayers of two types of nanoparticles. We have characterized capped nanoparticles that are anionic and cationic in nature. Electrostatic assembly of the nanoparticles has resulted in a monolayer of the particles. Use of suitable capping agents has enabled us to form a junction between two monolayers via electrostatic adsorption. Junctions between a pand an n-type nanoparticle (e.g., PbSe/CdSe, PbSe/CdS, PbS/ CdSe, and PbS/CdS) have exhibited rectification. Reverse junctions, formed by inverting the sequence of adsorption of the nanoparticles, have yielded rectification in the opposite direction. Since the sequence of capping materials has been the same in pn and np junctions, the results ruled out any contribution of the interfaces on the observed rectification. Control experiments with the individual components of the junctions yielded no rectification. The requirement of using a p- and an n-type nanoparticle in forming a junction has been established by presenting symmetric I-V characteristics (no rectification) from a junction between two p-type nanoparticles (PbSe/PbS) or two n-type nanoparticles (CdSe/CdS). Acknowledgment. K.M. acknowledges CSIR Junior Research Fellowship 9/80(491)/2005-EMR-I, Roll 509342). The Department of Science & Technology, Government of India financially supported the work through Ramanna Fellowship SR/S2/RFCMP-02/2005. References and Notes (1) Kovtyukhova, N. I.; Martin, B. R.; Mbindyo, J. K. N.; Smith, P. A.; Razavi, B.; Mayer, T. S.; Mallouk, T. E. J. Phys. Chem. B 2001, 105, 8762-8769. (2) Lee, D.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 23052312. (3) Lin, Y. W.; Tseng, W. L.; Chang, H. T. AdV. Mater. 2006, 18, 1381-1386. (4) Mohanta, K.; Majee, S. K.; Batabyal, S. K.; Pal, A. J. J. Phys. Chem. B 2006, 110, 18231-18235. (5) Mohanta, K.; Batabyal, S. K.; Pal, A. J. Chem. Mater. 2007, 19, 3662-3666. (6) Chen, W.; Grouquist, D.; Roark, J. J. Nanosci. Nanotechnol. 2002, 2, 47-53. (7) Landes, C.; El-Sayed, M. A. J. Phys. Chem. A 2002, 106, 76217627. (8) Poznyak, S. K.; Osipovich, N. P.; Shavel, A.; Talapin, D. V.; Gao, M. Y.; Eychmuller, A.; Gaponik, N. J. Phys. Chem. B 2005, 109, 10941100. (9) Akamatsu, K.; Tsuruoka, T.; Nawafune, H. J. Am. Chem. Soc. 2005, 127, 1634-1635. (10) Zhou, Y. X.; Gaur, A.; Hur, S. H.; Kocabas, C.; Meitl, M. A.; Shim, M.; Rogers, J. A. Nano Lett. 2004, 4, 2031-2035.

3238 J. Phys. Chem. C, Vol. 112, No. 9, 2008 (11) Harnack, O.; Pacholski, C.; Weller, H.; Yasuda, A.; Wessels, J. M. Nano Lett. 2003, 3, 1097-1101. (12) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617-620. (13) Kovtyukhova, N. L.; Mallouk, T. E. AdV. Mater. 2005, 17, 187192. (14) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848-7859. (15) Lowman, G. M.; Nelson, S. L.; Graves, S. M.; Strouse, G. F.; Buratto, S. K. Langmuir 2004, 20, 2057-2059. (16) Lesser, C.; Gao, M.; Kirstein, S. Mater. Sci. Eng., C 1999, 8-9, 159-162. (17) Ariga, K.; Hill, J. P.; Ji, Q. M. Phys. Chem. Chem. Phys. 2007, 9, 2319-2340.

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