Communication pubs.acs.org/cm
Thickness-Controlled Synthesis of Colloidal PbS Nanosheets and Their Thickness-Dependent Energy Gaps Ghadendra B. Bhandari,† Kamal Subedi,† Yufan He,‡,§ Zhoufeng Jiang,†,§ Matthew Leopold,† Nick Reilly,† H. Peter Lu,‡,§ Alexey T. Zayak,†,§ and Liangfeng Sun*,†,§ †
Department of Physics and Astronomy, ‡Department of Chemistry, and §Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, United States S Supporting Information *
I
match each reaction temperature. The reaction temperature is set at 20 °C above the boiling point of each cosolvent to avoid degraded quality (Supporting Information 3) or formation of QDs30 instead of NSs. At this point, the selection of the choloroalkane cosolvents is determined by the reaction temperature. The effect due to the nature of the cosolvent is still under investigation. Electron microscopy shows that each NS has a 2D structure with a lateral size of a few hundred nanometers (Figure 1). The NSs tend to form clusters with multiple layers due to the strong
n colloidal quantum-dots (QDs) based optoelectronic devices, the charge transport is limited by the low efficient phonon-assisted tunneling among the neighbor QDs. Although short organic ligands,1−3 inorganic ligands,4 and atomic ligands5,6 have been developed to replace the original ligands, the boundaries of QDs still hinder the charge transport. Making two-dimensional (2D) single-crystal nanosheets (NSs) can effectively reduce these hindrances7,8 yet retain the quantum confinement in one dimension. On the other hand, the 2D structure results in novel properties, such as highly efficient carrier multiplication,9 enhanced optical absorption and radiative recombination,10 and extremely narrow emission spectra.10,11 Methods to grow colloidal 2D NSs of cadmium salts (CdS, CdSe, and CdTe),10−16 lead salts (PbS, PbSe, and PbTe),7,8,17−22 and others23−26 have recently emerged. In the synthesis of nanostructured lead-salts that have cubiccrystal symmetry, the interaction among the crystal surfaces, ligands, and cosolvents plays a critical role on their shapes.7,8,27,28 Recently, it has been demonstrated that the chlorine-containing cosolvents can drive oriented attachments of PbS QDs to form ultrathin NSs.7,8 However, the strategies for controlling the NS thickness have not yet been established, and the structure-related intrinsic material properties are not well understood. In this Communication, we report thicknesscontrolled syntheses of colloidal PbS NSs with widely tunable thickness from 1.2 to 4.6 nm. We find that the dependence of the energy gap on the thickness (L) follows a 1/L law and demonstrate experimentally the difference of confinement energy in zero-dimensional (0D) and 2D systems. Built upon the 2D oriented attachment method developed in Weller’s group7 and Klinke’s group,8,9 our method (Supporting Information 1) explores the effect of different synthetic conditions on the thickness of NSs. We find that the thickness can be systematically tuned through changing the reaction temperature and the corresponding chlorine-containing cosolvents. Interrupted growth shows that the NSs are formed through 2D attachment of QDs (Supporting Information 2). Since the reaction temperature can control the size of the PbS QDs,29 it is expected to control the thickness of NSs as well. In our syntheses, the reaction temperature is tuned from 90 to 175 °C to synthesize NSs of different thickness. A series of chloroalkane cosolvents of different boiling pointschloroform (61 °C), 1,2-dichloroethane (DCE) (84 °C), 1,1,2-trichloroethane (TCE) (110−115 °C), 1,2-dichlorobutane (DCB) (125 °C), and 1,2,3-trichloropropane (TCP) (156 °C)are used to © 2014 American Chemical Society
Figure 1. TEM images of PbS NSs synthesized with (a) chloroform, (b) DCE, (c) TCE, (d) DCB, and (e) TCP as cosolvents. (f) Highresolution TEM image of a PbS NS (synthesized with TCE) showing a well resolved lattice plan of 0.297 nm spacing corresponding to the (002) and (020) plane of the PbS rock-salt structure. Inset, the selected-area diffraction pattern with corresponding diffraction spots. Received: July 11, 2014 Revised: September 1, 2014 Published: September 8, 2014 5433
dx.doi.org/10.1021/cm502524z | Chem. Mater. 2014, 26, 5433−5436
Chemistry of Materials
Communication
sheet−sheet interaction. The contrast between the NSs and the background in the transmission electron microscopy (TEM) images increases from (a) to (e) in Figure 1, indicating thicker sheets at higher reaction temperatures. The lead and sulfur atoms of the NSs form a square lattice (Figure 1f), reflecting the cubic structure of a PbS crystal. This square-lattice structure, as viewed from the top of the NS, reveals that the top surface of the NS is a {100} facet. Selected-area electron diffraction pattern of individual sheets confirms this observation, showing a single-crystal galena structure in [100] orientation (Figure 1f, inset). The atomic percentages of the NSs, measured in situ using an energy-dispersive X-ray spectroscopy (EDS) detector mounted on the TEM, reveal the Pb:S stoichiometric ratio of ∼1:1 (Supporting Information 4). The PL from the PbS NSs (Supporting Information 5) is observed in the near-infrared region (Figure 2a). The PL peak
Table 1. Synthesis Conditions for PbS NSs and Their Corresponding Optical and Thickness Characterizationsa T [°C]
cosolvent
90 104 130 160 175
chloroform DCE TCE DCB TCP
PL peak [nm] 1470 1530 1840 2000
± ± ± ±
125 150 150 150
absorption edge [nm] 1550 1525 1850 1900 2175
± ± ± ± ±
150 125 150 170 100
thickness [nm] 1.2 1.3 2.3 4.2 4.6
± ± ± ± ±
0.2 0.2 0.4 1.3 0.5
a
T is the reaction temperature. Each PL peak is expressed as peak wavelength ± half width at half-maximum. Each absorption edge is determined by the lowest energy peak in the differentiated absorption curve and expressed in the same format as the PL. The thickness is expressed as mean ± standard deviation.
large areas (Figure 3a, Supporting Information 7), enabling accurate thickness measurements.
Figure 2. (a) PL spectra of the PbS NSs synthesized at 90 °C (solid), 104 °C (dashed), 130 °C (dotted), and 160 °C (dashed−dotted), respectively. Each spectrum is normalized to its maximum intensity. (b) Optical absorption spectra of the PbS NSs synthesized at different reaction temperatures. The curves are vertically shifted for clarity. The dotted line is to guide the eyes to the shift of the absorption edge. (c) The PL peak (dashed line) overlaps the absorption edge (solid line) of the PbS NSs synthesized at 104 °C.
Figure 3. (a) Topography of the sample of PbS NSs synthesized with TCE cosolvent deposited on top of a silicon wafer. (b) The height profile of the layer of the PbS NS marked by a straight line “1” in the topography. (c) The histogram showing the distribution of the thicknesses of the NSs synthesized with TCE cosolvent.
shifts from short to long wavelengths as the reaction temperature increases. The PL of the NSs synthesized at higher temperatures shows lower intensity, and it is too weak to be detected when the reaction temperature is 175 °C. A similar trend was also observed in CdS nanoplatelets.10 The optical absorption spectra of the NSs are measured with our homebuilt system where an integrating sphere is used to suppress the effect of the scattered light31 from the NSs (Supporting Information 6). The spectrum of each NS (Figure 2b) shows a clear absorption edge which shifts to longer wavelength as the reaction temperature increases (except the increase from 90 to 104 °C, where the absorption edge stays nearly the same). The PL peaks overlap the absorption edges (Table 1). For instance, both the PL peak and the absorption edge of the NSs synthesized at 104 °C are located around 1530 nm (Figure 2c), indicating an intrinsic emission from the NSs. The energy gap of each NS is then derived. To find out the dependence of the energy gap on the thickness, an accurate thickness of each NS is required, and it is measured using atomic force microscopy (AFM). Each assynthesized NS has two capping layers of oleic acid (OA) ligands which have thicknesses ranging from 1.7 to 2.4 nm.32−34 To accurately measure the thickness of the PbS core, the OA ligands are replaced by 3-mercaptopropionic acid (MPA) ligands. The completeness of the ligand-exchange is confirmed by Fourier transform infrared spectroscopy (FTIR) (Supporting Information 7). After ligand-exchange the NSs spread out in
In Figure 3, the PbS NSs synthesized with TCE are used as a model system to demonstrate the thickness measurement. Single layer NSs can be identified around the edge of each NS cluster (Figure 3a). The height profile of one layer of NS obtained from the topography shows uniform thickness (Figure 3b). The average and standard deviation of the thickness (Figure 3c) are calculated through statistics of 45 height profiles. The MPA ligands likely adopt a lying-down configuration, and their contribution to the thickness of the sheets is estimated to be less than 0.2 nm (Supporting Information 8). All the NS samples were measured by AFM (Supporting Information 8), and their results are shown in Table 1. As a comparison, the thickness of the NSs (synthesized with TCE) before ligand-exchange is also measured (Supporting Information 9). The thickness of PbS is about 2.4 nm if each OA layer is assumed to be 1.8 nm thick. This result is close to 2.3 ± 0.4 nm obtained after ligand-exchange. The dependence of the energy gap on the thickness of the NS can be better understood by plotting all the data together in a single graph (Figure 4). The dependence of the energy gap on the thickness is fitted well by the following equation 1 Egap(L) = Egap(∞) + (1) 0.99L + 1.18 where Egap is the energy gap (eV) and L is the thickness (nm) of the NSs. At room temperature, the energy gap of the bulk 5434
dx.doi.org/10.1021/cm502524z | Chem. Mater. 2014, 26, 5433−5436
Chemistry of Materials
Communication
The absorption spectra of the PbS NSs show clearly step edges but not sharp excitonic peaks as observed in the cadmium-salts nanoplatelets.10 This is partially due to the thickness dispersity in the PbS NSs as well as the large dielectric constant of PbS. The electron−hole Coulomb interaction in PbS NSs is expected to be canceled by the self-interaction energies as in 1D PbSe nanorods,41 which is different from that in cadmium-salt nanoplatelets.10 Consequently, the optical energy gap of PbS NSs is mainly determined by the quantum confinement and the bulk energy gap, consistent with our spectroscopic results. This is in contrast to the cadmium-salt nanoplatelets where the energy gap is significantly affected by the electron−hole Coulomb interaction.10 In summary, we have developed a method to synthesize PbS NSs with controlled thicknesses and demonstrated their thickness-dependent PL and optical absorption. The energy gap dependence on the thickness is found to be 1/L instead of 1/L2, consistent with DFT calculations. The synthetic method and the electronic structure we reported here are important for the applications of PbS NSs as well as the fundamental understanding of the colloidal 2D semiconductor systems.
Figure 4. Dependence of energy gaps (solid dots) of the PbS NSs on their thicknesses. Vertical error bars indicate the errors in the energy gap derived from PL and absorption data. Horizontal error bars indicate the errors in the thickness measured by AFM. Square data are calculated using DFT. Inset: the dependence of the energy gaps of the PbS QDs on their diameters. Solid triangle data represent the experimental data from the literature;39 solid square data represent the calculated results using DFT. The solid curve is the fitting curve39 for the experimental data.
■
ASSOCIATED CONTENT
* Supporting Information S
PbS (Egap(∞)) is 0.41 eV. Fitting the data using a quadratic expression (aL2 + bL + c) or (aL2 + b) in the denominator of eq 1 results in a negative value of a in aL2 + bL + c which is physically meaningless or a bad fit with aL2 + b (Supporting Information 10). This 1/L dependence of the energy gap is unusual since a simple effective mass theory predicts a 1/L2 dependence.35 This difference is likely due to the breakdown of the effective-mass approximation when the thickness of the NS is only a few nanometers. A similar energy-gap dependence was also observed in very small PbS and PbSe QDs35−37 and was predicted for PbSe quantum wells by tight-binding calculations.38 This energy-gap dependence is confirmed by density functional theory (DFT) calculations (Supporting Information 11). The calculated data (Figure 4, squares) fall closely on the curve determined by eq 1. The calculated energy gaps (Figure 4, inset, solid squares) of the PbS QDs using the same method are consistent with those from experiments39 (Figure 4, inset, solid triangles). Both sets of data for QDs fall on the fitting curve determined by the experiments: Egap = 0.41 + 1/ (0.0252d2 + 0.283d),39 where d is the diameter of the QD. The confinement energy in 2D PbS NSs is about 14% to 43% of that in 0D QDs of the same confinement size from 1 to 8 nm (Figure 4 and inset), consistent with tight-binding calculations for PbSe QDs and quantum wells.38 This explains the small energy gap (
