Abstract
A self-assembly approach to lead selenide (PbSe) structures that have organized across multiple length scales and multiple dimensions has been achieved. These structures consist of angstrom-scale 0D PbSe crystals, synthesized via a hot solution process, which have stacked into 1D nanorods via aligned dipoles. These 1D nanorods have arranged into nanoscale 2D sheets via directional short-ranged attraction. The nanoscale 2D sheets then further aligned into larger 2D microscale planes. In this study, the authors have characterized the PbSe structures via normal and cryo-TEM and EDX showing that this multiscale multidimensional self-assembled alignment is not due to drying effects. These PbSe structures hold promise for applications in advanced materials—particularly electronic technologies, where alignment can aid in device performance.
1. Introduction
The interest in nanoscale materials stems from the fact that their properties (electrical, optical, chemical, and mechanical, among others) are a function of their size, chemical makeup, and underlying structure. Oriented two-dimensional (2D) materials [1, 2], tunable nanostructures [3–8], and quantum dots [9–11] have gained much attention in recent years due to their promising applications in a number of fields including energy [12], sensors [13, 14], and biomedicine [15]. Functional and multifunctional semiconducting nanoparticles are of considerable interest, as the bandgaps of these materials are tunable to a wide range of absorbances and emissions. Narrow-bandgap IV–VI semiconducting lead selenide (PbSe) structures have shown particular promise in third-generation solar cells, infrared optoelectronics, low cost microelectronics, and biological imaging and biosensor systems [16, 17]. Hence, recent work on the colloidal and composite synthesis, fabrication, and processing of PbSe structures has gained interest among a wide range of disciplines.
In this work, we present a PbSe structure self-assembled system where the structures have aligned across multiple length scales and dimensions (Figure 1), where the term self-assembly refers to the autonomous organization of components into patterns or structures without human intervention [18]. Although self-assembly originated in the study of molecules, it is a strategy that is applicable at all scales [18], as shown through this work. These structures consist of angstrom-scale 0D PbSe crystals, which have stacked into 1D nanorods. These 1D nanorods have arranged into 2D nanosheets that further aligned into larger 2D microscale planes. These 2D structures differ from randomly distributed nanorods, nanosheets, nanocrystals, and so forth and may hold promise due to their potentially enhanced properties. These oriented structures could easily be implemented into photovoltaic devices for increased efficiency—via their multiple exciton generation [19–23]. PbSe structures, such as these, are also of special interest for electrical applications due to their easily controlled alignment and connectivity [24].
2. Materials and Methods
Tri-n-octylphosphine (TOP), selenium powder, lead acetate trihydrate, oleic acid, n-tetradecylphosphonic acid, and diphenyl ether were purchased from Sigma-Aldrich for the synthesis. All syntheses were done under dry nitrogen. The synthesis of the PbSe structures followed an existing hot solution synthesis route [25] with modifications. A 1.0 M stock solution of TOPSe was prepared by adding 7.86 g of selenium to 100 mL of TOP and mixing for 2 hours at 50°C. Lead oleate was formed in situ by mixing 0.76 g of lead acetate trihydrate with 2 mL of oleic acid in 10 mL diphenyl ether and heating for 30 minutes at 150°C under nitrogen flow via a bubbler. The lead oleate solution was then cooled to 60°C and 4 mL of TOPSe was added to this solution (lead oleate-TOPSe). In a separate beaker, 0.2 g of n-tetradecylphosphonic acid was added to 15 mL of diphenyl ether and heated to 250°C with vigorous stirring. The lead oleate-TOPSe solution was then added to the solution of n-tetradecylphosphonic acid in diphenyl ether. The final solution was heated for 50 seconds at 250°C and then cooled to room temperature slowly. While the solution cools, the liquid turns cloudy, indicating the formation of the PbSe structures. Finally, 31 mL of hexane is added to this solution. These PbSe structures were then centrifuged and resuspended in different solvents (chloroform, water, and tetrahydrofuran). These are compared to the same structures with the capping agent (TOP) removed by a heat treatment [26], where the nanoparticles are heat-treated at 400°C in air for one hour. The procedures followed were in accordance with the ethical standards of the institutions involved.
Normal and cryogenic transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) of the PbSe structures were conducted on a JEOL JEM 2100 instrument operated at 200 keV. Sample preparation for cryo-TEM was done using a Vitrobot automated vitrification robot on lacey-carbon grids. Normal TEM samples were prepared on carbon-formvar 300-mesh copper grids. Here, an aliquot of PbSe in hexane was pipetted onto a grid and allowed to dry in open air.
3. Results and Discussion
3.1. Transmission Electron Microscopy
The TEM micrographs of the PbSe structures with TOP, shown in Figure 2, show the alignment of the individual 2D sheets, as well as the bulk arrangement of the structure into a microscale 2D plane. In Figure 2(a), the individual structures, made up of stacked PbSe nanorods, seem to be further aligned in bulk. It can be observed that this bulk arrangement seems to also align in a particularly ordered pattern, which could serve as an easily distributed array of further self-assembled structures. Figure 2(b) shows a higher magnified micrograph in which the individual structures, which make up the sheets, can be seen. The average length of the structures is distributed in a narrow range of 100 nm–150 nm, which can also be seen in Figure 2(c). The evenly spaced aligned nanorods can be seen in Figure 2(d), where the individual structures are made up of many much smaller nanorods. Previous results [25] show that the parameters controlling the formation in this synthesis route are the reaction temperature, reactant concentrations, and the concentration/composition of stabilizing agents, where the injection and growth temperatures show the greatest influence on the formation of the nanostructures. Figure 2(d) also shows some larger speckled dots, and, based on the EDX analysis, these dots can be attributed to undissolved or unreacted lead and selenium, as seen in other nanosemiconductor synthesis models [27].
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The possibility that this stacking was being caused by drying of the samples on the grid was examined using cryogenic transmission electron microscopy. Stacked structures are seen in Figures 3(a) and 3(b), which proves that the alignment of the structures is not due to drying and exists in solution. Figure 3(b) is a high-magnification micrograph that shows that the nanorods, in terms of both spacing and size distribution, are uniform with an average of nm in diameter and an average center-to-center spacing of nm. Measurements were made using free ImageJ open-source software (available on the NIH website: http://rsbweb.nih.gov/ij/).
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3.2. Energy Dispersive X-Ray Spectroscopy
The energy dispersive X-ray spectroscopy (EDX) analysis is shown in Figure 4. The PbSe structures exhibit both major and minor peaks of lead (Lα = 10.5 keV, Mα = 2.3 keV) and selenium (Kα = 11.2 keV, Lα = 1.3 keV). This is characteristic of the presence of PbSe. The copper, carbon, and oxygen peaks can be attributed to the carbon-formvar copper TEM grid.
PbSe structures that have aligned across multiple length scales and dimensions were synthesized and characterized through TEM and EDX. Given their nanoscale size and reported multiscale multidimensional orientation, these PbSe structures can serve as inorganic semiconductors for advanced materials. This differs from typical PbSe structures, which exhibit absorption over a wide range of the infrared spectrum [28] where spectral irradiance is sparse and is divided up by many H2O and CO2 absorption bands. Although self-assembly originated in the study of molecules, it is a strategy that is applicable at all scales [18], as shown through this work.
The observed system is an example of static self-assembly—one in which the system is at global or local equilibrium and does not dissipate energy [18]. One-dimensional nanostructure growth has been studied for a number of years, having a number of assembly mechanisms [29–31]. Dipole-dipole interactions are the most likely candidate for the driving force directing the 0D PbSe crystals to 1D rod assembly mechanism [25, 32], reinforced by the isotropy of the crystals—each having six identical facets. Previous models have also shown that this system should have permanent nonzero dipole moments—up to 89%—with a prediction of the largest dipole moment along the 〈100〉 axis [25]. Similar stacking of 1D rods to 2D sheets has been shown in other systems [33], where the interfacial interactions play a key role in the 2D alignment. Similar mechanisms are suspected to be at play in the current system; however, further investigation is needed. If the system is found to unalign upon removal of TOP, then it is possible that alignment is due to directional short-ranged attraction or molecular self-assembly, such as hydrogen and coordination bonding, Van der Waals interactions, electrostatic interactions, or hydrophobic interactions.
TEM of the TOP-removed structures has yet to be completed but is expected to yield unaligned rods. If this hypothesis is consistent with the results obtained, it could be speculated that the structures are aligning due to enthalpic or entropic effects between the hydrophobic chains of the capping agent, TOP, or neighboring structures. This case would be an example of meso- or macroscopic interactions (i.e., gravitational interactions and external electromagnetic, capillary, and entropic interactions). This has been observed as 1- and 2-dimensional structures in some nanoparticle systems of differing shapes and sizes [34–36] and has been attributed to the combination of size, directional attraction, and hydrophobic and electrostatic interactions of the system [37].
Further experimentation to be investigated, with the confirmation that stacking continues after the removal of TOP, could include a magnetic or polarization study to see whether the alignment is due to a dipole in the PbSe structures. Magnetically and photomagnetically forced alignment and positioning have been displayed in a number of quantum dot and nanorod anisotropic electrostatic systems [38–43]. However, all are induced by taking advantage of dipolar interactions upon applying a magnetic force and/or doping the nanocrystals with a magnetically sensitive cation; therefore, these systems do not spontaneously self-assemble. The authors also do not suspect any influence of the carbon-formvar copper TEM grid on the assembly.
4. Conclusions
In conclusion, the presented PbSe self-assembled system—where the structures have aligned across multiple length scales and dimensions—has been characterized and explored. It has been shown that these structures consist of angstrom-scale 0D PbSe crystals, which have stacked into 1D nanorods and then are further arranged into 2D nanosheets. These 2D nanosheets then align into larger 2D microscale planes. Self-assembled ensembles such as these provide a solution to the fabrication of ordered aggregates from components with sizes ranging from the angstrom to micron scale. The interest here lies in the notion that these components typically are found between the sizes that can be manipulated by chemistry and those that can be manipulated by conventional macro techniques. This range of sizes is extremely important for the further development of nanotechnology in the areas of inorganic or biomimetic syntheses and requires further investigation as an entire field.
In continuing this work, thermal analysis will be conducted to further verify that the TOP has been removed. This will investigate the thermal properties of the PbSe structures as well, but it should be noted that—being entirely inorganic—any photodegradation of these structures would be insignificant compared to the materials used in common hybrid organic/inorganic systems. Future work will also focus on a tighter control of the alignment, size, and shape of the PbSe structures, as well as the determination of the assembly mechanism, further advanced characterization, and integration into devices. In electronics, namely, nanoelectronics, where the importance of single-particle interactions is magnified, alignment and symmetry are of the utmost importance, making this unique self-assembled alignment advantageous for enhanced optical, magnetic, and/or electrical properties, particularly in advanced materials.
Competing Interests
The authors declare that they have no competing interests.
Acknowledgments
The authors would like to acknowledge the NASA funded Rhode Island Space Grant Consortium, the University of Rhode Island Transportation Center, and the Honda Initiation Grant for their financial support.