The synthesis of nanomaterials is the central component of our groups strength. Mapping materials properties with size is possible when samples of nanocrystals are monodisperse in size, shape, composition, internal structure, and surface chemistry. Once isolated a diverse set of structural and chemical probes must be combined to characterize these samples. Optical, electrical, and magnetic studies of nanocrystal samples reveal the unique size-dependent properties of materials in this intermediate, nanometer size regime between molecular species and bulk solid. Figure 1 provides a schematic of the key elements I developed in nanocrystal synthesis, separation, and assembly into nanocrystal solid films.
This approach has enabled production of semiconductor quantum dots and nanowires, metallic and metal oxide magnetic nanostructures, and semimetal and noble metal nanoparticles. Synthesis of II-VI semiconductors (1) include ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgSe and IV-VI semiconductors (6) include PbS, PbSe, and PbTe. Semimetals such as Bi nanoparticles are part of the current tool set. The preparation of monodisperse Co, Ni, Fe, FePt, CoPt, CoPt3, Co/Ni, Fe2O3, Fe304, MnFe2O4 and ZnFe2O4 provide model magnetic nanoparticle systems (3, 5, 7, 8) while CoO, NiO and Cr nanoparticles have been isolated as antiferromagnetic models. My team also produces Au, Ag, Pd, and Pt nanocrystals on par with the best uniformity reported to date. Ferroelectric and highdielectric nanoparticle systems like BaTiO3 and SrTiO3 and a ferroelectric but require further refinement.
Figure 1 (Left) Schematic of (A) the high temperature solution synthesis of nanocrystals, (B) size-selective purification, (C) self-assembly of nanocrystals (D) to form ordered assemblies (superlattices). (Right) (Top) The evolution from discrete molecular to band-like energy levels in semiconductor nanocrystals and its effect on the size dependent fluorescence of CdSe nanocrystals. (Bottom) The figure depicts evolution from superparamagnetic to single domain and ultimately to multi-domain structure in magnetic nanocrystals. Curves show the size dependent blocking temperature and coercivity of monodisperse Co nanocrystals.
Although we have come a long way in the synthesis of nanomaterials, we are leveraging only a small fraction of the knowledge found in traditional inorganic and organic chemistry communities. Procedures implemented thus far have been rewarding, but the materials design could be much more powerful. I am excited about the potential to achieve greater understanding of nanoparticle growth mechanisms and materials exchange reactions at the surface. The chemical challenge to tailor the reactivity of precursors allow self limiting growth of nanoparticles a single shell of atoms at a time might even be possible. This should allow true atomic precision in the synthesis of a range of nanostructures to be achieved and the realization and potential of “artificial atoms”. We will in general, stay focused on solution phase routes but would enjoy expanding beyond standard high boiling organic solvents. An intriguing opportunity that could couple chemical engineering expertise to establish continuous nanocrystal production methods would overcome the present limitations of batch processing.
We will strengthen methods exploit difference in chemistry on selected facets to control shape and assist in the directional assembly of nanoparticles. Building on existing strengths in the preparations of core/shell nanoparticle systems will also allow us to combine a number of the chemistries that yielded the individual nanoparticle systems. The organic surface organic chemistry is also ripe for exploitation. Although the list of chemicals we have grafting to the surface of our particles is extensive. The real fun will start when electro-active, optically active or biologically active ligands are exchanged and modified on the surface with the same ease as our simple capping groups.
Detailed structural and chemical analysis of nanocrystal samples is essential to complement the synthetic efforts. Transmission electron microscopy (TEM) is the single most powerful technique for characterizing nanocrystals and their assemblies. We rely heavily on quick low-resolution TEM studies of ensembles of nanocrystals. Recently we have begun to exploit advances in image analysis software to speed the development of statistical descriptions of nanocrystal size, size distribution and shape in our samples. HRTEM imaging reveals the individual nanocrystal shape and internal structure. Elemental analysis by energy dispersive x-ray EDX analysis is the first essential step to determine particle composition. These microscopic observations are compared with wide angle and small angle X-ray scattering studies, which simultaneously probe large areas providing a statistical sample. Chemical spectroscopies, including infrared absorption and Raman help to develop a model of the organic capping layers. We hope to grow our activities and interactions to work more with solution phase and solid state NMR (9) to provide more chemical insight. We will work to unite the results of these diverse techniques through a single atomistic model of average NC structure and to develop a coherent description of the system.
The whole is greater that the sum of the parts. When atoms or molecules organize into condensed systems, new collective phenomena develop. Cooperative interactions produce the physical properties we recognize as characteristic of bulk materials. Like atoms or molecules, but in the next level of hierarchy, nanocrystals can behave as artificial atoms serving as the building blocks to new designer solids. Routes enabling controlled manipulation of nanocrystals into one-dimensional wires, through oriented attachment are advancing (see Figure 2). Techniques to produce well ordered 2D monolayers and 3D colloidal crystalline solids are also finding wide application (10,11). I have worked to show that the assembly of nanocrystals can enable design of new solid state materials and devices. We are excited to build a program here at UPenn that probes more deeply how interactions between nanocrystals give rise to new collective phenomena (12, 13) particularly electronic transport, photoconductivity, and magnetic exchange coupling, In addition magnetic arrays that could extend to the limit of storage with individual magnetic particles make compelling targets (3). Our preliminary exploration of nanowire based field effect devices motivates further exploration designs exploit the emerging shape control.
Figure 2 (Left) shows an image of the reaction used to produce the PbSe quantum dots and to form semiconductor nanowires. (Right) examples of different PbSe nanowire morphologies that can be isolated in high yield be adjusting the reaction temperature and the choice of organic stabilizer. (14)
Putting artificial atoms to work necessitates that we build upon a rigorous understanding of the physical properties of individual nanocrystals to allow the properties of coupled nanocrystals in the solids to be uncovered and harnessed in new devices. Engineering the size and composition of the nanocrystals and the length and chemical functionality of the matrix may be used to tune the electronic coupling of nanocrystal building blocks My has team pushed the design of nanocrystal arrays as models for scaling in magnetic storage media (3), explored high-energy density magnetic nanocomposites, and worked to fabricate magneto-resistive devices based on transport in nanocrystal solids (15, 16). Expanded control of interparticle coupling may allow more subtle tuning of the exchange interactions and interesting spin-dependent phenomena.
The most intriguing element of chemical control however is just now with in our grasp. Doping nanostructures has been a grand challenge that is only now yielding to synthetic pressures to achieve more conventional substitutional doping and charge transfer doping at the nanostructures interfaces is showing promise. We have used chemical doping of semiconducting nanocrystal solids is to fabricated into n and p channel field effect devices with surprisingly high mobilities (17). Initially insulating PbSe nanocrystal solids (quantum dot arrays, superlattices) can be chemically “activated” to fabricate n- and p-channel field effect transistors (FETs), while retaining the size quantization effects prized in the constituent building blocks (Figure 3). We demonstrated field effect electron and hole mobilities of 0.9 and 0.2 cm2/Vs, respectively, and current modulations (Ion/Ioff) of ~103-104. Chemical treatments engineer the interparticle spacing, electronic coupling and doping in the films while simultaneously passivating electronic traps. The electronic performance of these nanocrystal FETs’ compares favorably with more mature organic transistors and provides complementary circuit (CMOS) options that could enable a range of low cost, large area electronic, optoelectronic, thermoelectric, and sensing applications. Initial results on PbTe nanocrystal solids are surpassing our work on PbSe and further development of this system will be an early target. The PbTe system has added appeal as it weak electron phonon coupling and low bulk thermal conductivity make it a promising candidate for thermoelectric power generation.
Figure 3. (a) Optical absorption spectrum of a colloidal 8 nm PbSe nanocrystals in tetrachloroethylene. (b) TEM image of an array of 8 nm PbSe nanocrystals. Inset shows wide-angle electron diffraction from a 25 µm2 area of the nanocrystal film. (c) High resolution SEM cross-section of PbSe nanocrystal film forming the channel of a thin-film transistor, schematic shown in (d) where S and D are the gold source and drain electrodes. A highly doped Si substrate (G) was used as the back gate. (e) GISAXS pattern of PbSe nanocrystal film.
My team has championed modular approaches to multifunctional nanocomposites by focusing on binary crystallization of colloidal nanoparticles. The assembly of nanoparticles of two different materials into a binary nanoparticle superlattice in fact can provide a general and inexpensive path to a large variety of novel materials with precisely controlled chemical composition and tight placement of components. We have demonstrated the formation of more than fifteen different binary nanoparticle superlattice structures using combinations of semiconducting, metallic, and magnetic nanoparticle building blocks (18-21). Some examples are seen in Figure 4. At least ten of these colloidal crystalline structures are reported for the first time. We have also demonstrated that static electrical charges on sterically stabilized nanoparticles determine binary superlattice stoichiometry and, with additional contributions from entropic, van der Waals, steric, and dipolar forces, stabilize this rich variety of structures. This is just the tip of the iceberg for the design of multi-functional nanomaterials.
Figure 4. TEM images of the characteristic projections of the binary superlattices self-assembled from colloidal solutions of different nanoparticle mixtures. Insets show the unit cells of corresponding 3D structures. The superlattices are assembled from (a) 6.2 nm PbSe and 3.0 nm Pd; (b) 7.6 nm PbSe and 5.0 nm; (c) 13.4 nm γ-Fe2O3 and 5.0 nm Au; (d) 6.7 nm PbS and 3.0 nm Pd; (e) 6.2 nm PbSe and 3.0 nm Pd; (f) 5.8 nm PbSe and 3.0 nm Pd; (g) 7.2 nm PbSe and 4.2 nm Ag; (h) 6.2 nm PbSe and 3.0 nm Pd; (i) 7.2 nm PbSe and 5.0 nm Au; (j) 5.8 nm PbSe and 3.0 nm Pd; (k) 7.2 nm PbSe and 4.2 nm Ag; (l) 6.2 nm PbSe and 3.0 nm Pd nanoparticles. The scale bars are 20 nm except (d), (g) and (h) where the scale bars are 10nm.
We are working to keep the momentum on the assembly of new materials while bringing new talents to bear on the characterization of the properties the solids should possess. For example, the ability to combine nanoscale semiconductors and magnetic components raise the potential to engineer a host of magneto-optical and magneto-transport phenomena as the local magnetic fields of the magnetic nanocrystals act to Zeeman split the energy levels in the quantum dots. The opportunity to build solids with an intimate mixing of quantum dots and either Au or Ag nanocrystals could allow the strong electric fields from the noble metals surface plasmons to perturb the quantum dots energy levels. These effects might in turn be harnessed in optical switches, isolators, and other optoelectronic devices.
These advances motivate many technological applications but also raise the responsibility that as these novel materials prepare to leave the lab, their biological and electronic activity be much more carefully studied. What was a compelling choice at milligram quantities to aid in a particular scientific inquiry may look much less appealing when someone want to wants to paint your roof tiles with it to harvest energy or to supply you with a disposable display device. Collaborations to study the biological activity and health and safety impacts of the existing materials will be actively sought and even more environmentally friendly alternatives will be explored for potentials scale up. The core competencies of the research program I’m proposing could also stimulate effective collaborative interactions in bionanotechnology (22, 23) and sensor development.
Our exploration of nanocrystalline materials is allowing us to work at frontiers of materials chemistry, physics, engineering and even biology. We are convinced that the cutting edge research program we are building will provide excellent learning opportunities for new scientists. We will build vibrant collaborations been groups with traditional strengths in these disciplines. It is in the design and characterization of advanced materials that the importance of new interdisciplinary studies will be realized.
(1) “Synthesis and Characterization of Nearly Monodisperse CDE (E = S, Se, Te) Semiconductor Nanocrystallites,” CB Murray, DJ Norris and MG Bawendi, JACS 115 (19): 8706-8715, Sept. 22, 1993, ISSN: 0002-7863.
(2) “Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies,” CB Murray, CR Kagan and MG Bawendi, Annual Reviews of Materials Science 2000, Vol. 30, pp. 545-610.
(3) “Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices,” SH Sun, CB Murray, D Weller, L Folks and A Moser, Science 287 (5460): 1989-1992 March 17, 2000.
(4) “Synthesis of monodisperse cobalt nanocrystals and their assembly into magnetic superlattices,” SH Sun and CB Murray, JAP 85 (8): 4325-4330 Part 2A, April 15, 1999.
(5) “Monodisperse 3d Transition Metal (Co, Ni, Fe) Nanoparticles and Their Assembly into Nanoparticle Superlattices,” CB Murray, S Sun, H Doyle and T Betley, MRS Bulletin, 26 (12): 981-+, Dec., 2001.
(6) “Colloidal synthesis of nanocrystals and nanocrystal superlattices,” CB Murray, SH Sun, W Gaschler, H Doyle, TA Betley and CR Kagan, IBM J. of Research and Development 45 (1): 47-56, Jan., 2001.
(7) “Controlled Assembly of Monodisperse e-Cobalt-Based Nanocrystal,” S Sun, CB Murray and H Doyle, MRS Proceedings Vol. 577 Spring 1999 (Nov., 1999).
(8) “Magnetic, electronic, and structural characterization of nonstoichiometric iron oxides at the nanoscale,” FX Redl, CT Black, GC Papaefthymiou, RL Sandstrom, M Yin, H Zeng, CB Murray and SP O'Brien, JACS 126 (44): 14583-14599, Nov. 10, 2004.
(9) “Investigation of the Surface Morphology of Capped CdSe Nanocrystallites by P-31 Nuclear Magnetic Resonance,” LR Becerra, CB Murray, RG Griffin and MG Bawendi, J. of Chemical Physics 100 (4): 3297-3300, Feb. 15, 1994.
(10) “Self-Organization of CdSe Nanocrystallites into 3-dimensional Quantum-Dot,” CB Murray, CR Kagan and MG Bawendi, Science 270 (5240): 1335-1338 Nov. 24, 1995.
(11) “CdSe and CdSe/CdS nanorod solids,” DV Talapin, EV Shevchenko, CB Murray, A Kornowski, S Forster and H Weller, JACS 126 (40): 12984-12988, Oct. 13, 2004.
(12) “Long-range resonance transfer of electronic excitations in close-packed CdSe quantum-dot solids,” CR Kagan, CB Murray and MG Bawendi, PRB 54 (12): 8633-8643, Sept. 15, 1996.
(13) “Electronic energy transfer in CdSe quantum dot solids,” CR Kagan, CB Murray, M Nirmal and MG Bawendi PRL 76 (9): 1517-1520, Feb. 26, 1996.
(14) “Designing PbSe Nanowires and Nanorings Through Oriented Attachment of Nanoparticles” KS. Cho, DV. Talapin, W. Gaschler and C B Murray. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 127 (19): 7140-7147 MAY 18 2005
(15) “Spin-dependent tunneling in self-assembled cobalt-nanocrystal superlattices,” CT Black, CB Murray, RL Sandstrom and SH Sun, Science 290 (5494): 1131-1134 Nov. 10, 2000.
(16) “Magneto-transport in Magnetite Nanoparticle Arrays” Hao Zeng, C.T. Black, R.L. Sandstorm, P. Rice C.B. Murray, and Shouheng Sun
(17) “PbSe Nanocrystal Solids for n- and p-Channel Thin Film Field-Effect Transistors”
DV. Talapin and C. B. Murray Science Vol 310, No 5745, 86-89, 2005
(18) “Three-dimensional binary superlattices of magnetic nanocrystals and semiconductor quantum dots,” FX Redl, KS Cho, CB Murray and S O'Brien, Nature 423 (6943): 968-971, June 26, 2003.
(19) “Preparation and Characterization of binary nanocrystal superlattices.” In press JACS
E. V. Shevchenko, D. V. Talapin, C. B. Murray and S. O’Brien
(20) "Polymorphism in AB13 Nanoparticle Superlattices: an Example of Semiconductor-metal Metamaterials" EV Shevchenko, D. Talapin, SP O’Brien and CB Murray JACS 127 (24): 8741-8747 JUN 22 2005
(21) “Structural Diversity in Binary Nanoparticle Superlattices.” Submitted. E. V. Shevchenko, D V. Talapin, N A. Kotov, S O’Brien and C. B. Murray
(22) “Bio-functionalization of monodisperse magnetic nanoparticles and their use as biomolecular labels in a magnetic tunnel junction based sensor” J. Phys Chem B, 109 26; 1303-13035
SG Grancharov, H. Zeng, S. Sun SX Wang, S O’Brien, CB Murray, JR Kirtley, GA Held.
(23) “Cooperative assembly of magnetic nanoparticles and block copolypeptides in aqueous media,” LE Euliss, SG Grancharov, S O'Brien, TJ Deming, GD Stucky, CB Murray and GA Held, Nano Letters 3 (11): 1489-1493, Nov., 2003.