Infrared-Emitting Colloidal Nanocrystals - Wiley Online Library

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Mar 6, 2007 - Assembly, Spectroscopy, and Applications. Andrey L. Rogach,* Alexander Eychmüller, Stephen G. Hickey, and. Stephen V. Kershaw. From the ...
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A. L. Rogach et al.

Infrared emission DOI: 10.1002/smll.200600625

Infrared-Emitting Colloidal Nanocrystals: Synthesis, Assembly, Spectroscopy, and Applications Andrey L. Rogach,* Alexander Eychmller, Stephen G. Hickey, and Stephen V. Kershaw

From the Contents 1. Introduction............. 537 2. Synthesis, Assembly, and Spectroscopy of Infrared-Emitting Nanocrystals............ 538 3. Applications of InfraredEmitting Nanocrystals ................................ 550 4. Summary and Outlook ................................ 553

Keywords: Advanced research into IR-emitting nanocrystals provides significant promise in many fields of application.

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· colloids · infrared emission · nanocrystals · optics · semiconductors

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Semiconductor nanocrystals produced by means of colloidal chemistry in a solvent medium are an attractive class of nanometer-sized building blocks from which to create complex materials with unique properties for a variety of applications. Their optical and electronic properties can be tailored easily, both by their chemical composition and particle size. While colloidal nanocrystals emitting in the infrared region have seen a burst of attention during the last decade there is clearly a paucity of review articles covering their synthesis, assembly, spectroscopic characterization, and applications. This Review comprehensively addresses these topics for II–VI, III–V, and IV–VI nanocrystals, examples being HgTe and CdxHg1xTe, InP and InAs, and PbS, PbSe, and PbTe, respectively. Among the applications discussed here are optical amplifier media for telecommunications systems, electroluminescence devices, and noninvasive optical imaging in biology.

1. Introduction Nanostructured materials research is nowadays a rapidly growing field of science where the efforts of chemists, physicists, materials scientists, and, more recently, biological scientists and engineers have merged. Nanotechnology, which is defined as the creation and utilization of materials, systems, and devices through control of matter on the nanometer-length scale, is expected to become one of the main driving forces in materials research for the 21st century. An attractive class of nanometer-sized building blocks, from which to create ordered and complex materials with unique properties, are semiconductor nanocrystals (NCs) produced by methods of colloidal chemistry in a solvent medium. In NCs, atomic-like electronic energy levels are formed due to charge-carrier confinement in three dimensions. It is possible to tailor their optical and electronic properties to a given application using not only the intrinsic characteristics due to their chemical composition but also their perturbation due to their size and, in large ensembles, the distribution of sizes. The most striking example is the strong dependence of the energy gap (Eg) between conduction and valence levels when the particle diameter approaches the scale of the de Broglie wavelength of an excited electron within the particle. As the particle diameter is reduced the energy gap is blue-shifted (Figure 1) due to this so-called “quantum-confinement effect”, which may be modeled as a particle (an excited electron and its hole counterpart) in a three-dimensional (3D) box (the NC). At room temperature, the fluorescence corresponding to the recombination of the excited hole and electron in a single NC will have a finite width due to homogeneous line broadening. In addition, for ensembles of NCs produced by colloidal methods, due to the statistical distribution of particle sizes given by the growth process, there will be further broadening of the luminescence as depicted in Figure 1, due to the dependence of Eg on the particle size. small 2007, 3, No. 4, 536 – 557

Figure 1. Typical quantum-confinement-induced blue shift in conduction!valence level energy gap with decreasing NC diameter. Also shown are temperature-broadened line profiles for NCs at the center and half-width points of a distribution of particle sizes.

Most of the work carried out up to the end of 20th century on the synthesis and applications of semiconductor NCs dealt with materials emitting in the visible spectral range, and publications on IR-emitting NC materials ap[*] Dr. A. L. Rogach Photonics and Optoelectronics Group Physics Department and Centre for NanoScience (CeNS) Ludwig-Maximilians-Universit direction) and transverse dimensions of the Bohr ellipse for the lead chalcogenide NCs and note that the anisotropy becomes more pronounced as one progresses down the series (  1.4 for PbS,  2 for PbSe, and  12 for PbTe). Confinement effects are expected to be present in PbTe NCs that have a dimension on the 152-nm scale, the largest Bohr radius of any known semiconductor. Although the PbTe nanoparticles prepared in the study may possess a slightly larger polydispersity than that of the PbS and PbSe samples, this cannot account for the degree of broadening observed in the absorption spectra. Large-area assemblies of densely packed arrays of PbTe were formed by solution drop-casting from mixtures of hexane and octane,[106] and their electronic characterization (using the previously demonstrated chemical activation technique employed on highly doped silicon wafers with a 100 nm SiO2 gating layer) was described. The layers show a 9–10 order-of-magnitude increase in their conductance. Using grazing-incidence smallangle X-ray scattering (GISAXS), the average interparticle spacing was observed to decrease from 1.8 nm to 0.3 nm upon exposure to the hydrazine solution. In order to adequately test the validity of many of the hypotheses proposed to explain the experimental observations there have been a number of theoretical works on both clusters[133] and nanoparticles. Due to the interest in the lead chalcogenides there is a vast body of literature on their electronic structures and PbS has the distinction of being the first semiconductor for which the complete band structure was proposed.[134] In the work of Kang and Wise,[135] it was found that by employing a four-band envelope function formalism an accurate description of the experimental features observed for PbS and PbSe quantum dots in glasses could be accounted for. Wei and Zunger[136] applied local density approximation calculations to the band structure of the lead chalcogenide series and their alloys and proposed that the fact that the Pb s band lies below the top of the valance band and is capable of setting up coupling and level repulsion at the L point can account for many of the analogous properties observed in these materials, such as the analogous order of the bandgaps, negative optical bowing, negative pressure coefficients, and so on. Allen and Delerue have studied quantum confinement in PbSe quantum dots and quantum wells[137] using tight-binding calculations and have concluded that the high quantum efficiencies observed may be explained by the fact that the

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electronic states are mostly p type and as a consequence localized states are not created in the bandgap. They also predict that PbSe NCs of spherical and cubic geometry will possess similar confinement energies.

3. Applications of Infrared-Emitting Nanocrystals 3.1. Infrared-Emitting Nanocrystals for Telecommunications A particularly attractive market for colloidally produced IR NCs is in optical telecommunications. Most of the optoACHTUNGREelectronic activity in this market is in IR components operating at wavelengths ranging from approximately 1200– 1600 nm, though at present most commercial activity centers on bands around 1550 nm. This is largely due to the historical development of silica fibers as transmission media for high-speed (e.g., tens of GBit per second) digital optical signals. The majority of the currently deployed standard communications fibers have an IR absorption spectrum similar to that shown in Figure 20. Almost complete removal of the extrinsic hydroxyl absorption peaks is now possible, opening up a further range of IR wavelengths between 1300 and 1500 nm for potential use.

Figure 20. Absorption spectrum of a standard communication system fiber (heavy curve). Overtone and combination bands of vibrational modes (n1 and n3) of included hydroxyl groups dominate and lead to operation in two principal transmission windows at around 1300 nm and 1550 nm. Recent innovation in fiber processing has led to almost complete removal of these bands, giving a spectrum that closely follows the underlying Rayleigh scattering limit. However such “water-free” or “zero water peak” fibers have not yet been widely deployed. Reprinted with permission from Ref. [3a]. Copyright 2000, IUPAC.

The biggest single cost of providing a new fiber communications link is the capital expenditure of excavation and physical installation of the fiber itself. This has led to the development in the telecommunications world of two standards for the simultaneous carrying of several high-data-rate optical signals, each at different wavelengths on a single fiber. This approach increases the data-carrying capacity of

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the fiber and postpones the point at which further physical fiber must be added. Dense wavelength division multiplexing (DWDM) has been an integral process for most large telecom providers for a number of years and allows for multiple channels over a range of approximately 30 nm in the 1550 nm region. This is compatible with erbium (ion) doped silica fiber optical amplifiers, which can regenerate signals after transmission and other losses (e.g., signal splitting). Coarse wavelength division multiplexing (CWDM) is a more recent innovation, which spans a range of  1280 nm– 1620 nm and calls for more generously spaced operating wavelengths (20 nm apart) in an effort to reduce component costs. Whilst there are currently well-developed technologies for receivers (high-speed photodiodes), transmitters (lasers with external high-speed modulators), and optical amplifiers for  1550 nm operation, there are opportunities for technologies such as colloidal NCs in, for example, optical switching, wavelength change, optical amplification outside the erbium-doped-fiber amplifier window, and other optical signal processing functions. A substantial amount of development is required before NC/polymer-film technology is suitable for commercialgrade optical waveguide component production. However, initial steps have been made with the fabrication of multimode optical waveguides on silica substrates, produced by plasma etching spin-coated HgTe NC doped polymer films, as shown in Figure 21 a. Figure 21 b shows the IR fluores-

Figure 22. PL spectra of HgTe NCs from a stripe-pumped 3-mm-thick slab waveguide for a series of increasing stripe lengths. The pump wavelength in each case was 980 nm.

length edge of the PL spectrum a substantial enhancement of the spectrum was seen in this vicinity. Note also that the long-wavelength part of the spectrum extends several hundred nanometers into the IR region, covering most of the telecom spectrum, and over much of that range the photoluminescence intensity is almost flat. The length dependence of the PL intensity at a given wavelength will show if any gain is present; if so, it can be used to estimate the modal gain that is, including losses due to self-absorption, scattering, and lack of optical confinement, in the plane of the slab. Provided scattering due to sidewall roughness is not substantial and that appropriate guide dimensions Figure 21. a) Metal-coated HgTe NC-doped polymer waveguides fabricated by reactive-ion etching of a and refractive indices are spin-coated film on a silica substrate. b) IR emission from the end faces of a pair of sectioned multiused, the corresponding mode HgTe NC-doped polymer waveguides. c) Cross-sectional structure of multimode waveguides in (a). figure for a vertically and Optical confinement of the luminescence is mainly in HgTe NC-doped polymer ridges, with some leakage horizontally confined ridge into the slightly lower refractive index epoxy adhesive layer. waveguide should be higher. The PL versus stripe length plots for 1000, 1300, and 1500 nm (Figure 23) show an exponential initial rise indicatcence emitted from the sawn end faces of a pair of multiing optical gains of 6.9, 4, and 4.4 cm1, respectively. Since mode waveguides when the guides were transversely pumped with a 980 nm laser diode. The waveguide crossthe PL is nearly flat between 1300 and 1500 nm, the plots at sectional structure is shown in Figure 21 c. these wavelengths are virtually superimposed. For longer Luminescence spectra of the IR emission of HgTe NCs guide lengths the plots become linear as losses balance the from the edge of a slab (2D) waveguide have been oboptical gain available, bearing in mind that only a small proserved, as shown in Figure 22. The graph shows spectra for portion of the pump beam is absorbed when pumping a thin optically pumped stripes (approximately 200 mm in width) film transversely. Longer guides will therefore see a depleof progressively increasing lengths. In each case the stripe tion of the available amplification as the amplified signal started at the edge at which the output light from the guide reaches a saturation level. At the longest guide lengths the was collected; a pump wavelength of 980 nm was used. plots start to reduce in steepness as losses begin to dominate Since the pump wavelength overlapped with the short wavethe saturated gain. small 2007, 3, No. 4, 536 – 557

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Figure 23. Length dependence of PL intensity from a HgTe NC-doped slab waveguide at 1000 (*), 1300 (*), and 1500 nm ( ! ). Data for 1300 and 1500 nm are virtually superimposed. Optical gains are 6.9, 4, and 4.4 cm1, respectively. Note that these are modal gain coefficients under small signal and weak (absorbed) pump power conditions.

3.2. Near-Infrared LEDs based on Composites of Semiconductor NCs and Polymers or Small Organic Molecules The advantages of the rapidly developing organic lightemitting diode (OLED) technology[138] can be combined with the attractive properties of semiconductor NCs, whose emission color and electron affinity can be finely controlled, not only by the material choice but also by the particle size within a single synthetic route. Because of the limited choice of polymers and organic dyes emitting in the range of the telecommunications windows (1.3 and 1.55 mm), the use of IR-emitting NCs in hybrid organic–inorganic LEDs provides particularly attractive application potential. The first paper on such LEDs appeared in 2002 and reported on single-layer blend devices.[139] These were based on composite mixed films of core/shell InAs/ZnSe NCs and a conducting polymer – either poly(2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene) (MEH-PPV) or poly(9,9-dehexylfluorenyl-2,7-diyl)-co-(1,4-{benzo-(2,1’,3)thiadiazole}) (F6BT) spin-cast from toluene solutions. The devices showed broad emission in the range of 1.3 mm, which was attributed to the inhomogeneously broadened spectrum of the ensemble of differently sized NCs, with a rather high turnon voltage of 15 V and a remarkably high external quantum efficiency reaching 0.5 % at high operating voltages (Figure 24). The turn-on voltage increased in a series of LEDs with increasing volume fraction of NCs in the polymer, which is consistent with trapping of carriers at the NC sites and thus trap-limited transport in the blend. The quenching of both the PL and the electroluminescence (EL) of the host polymer matrix, however, also pointed to a possible role of energy transfer from the polymer host to the NCs. In a subsequent report,[140] EL spectra of single-layer devices based on mixtures of PbS NCs with MEH-PPV or poly(2-(6-cyano-6’-methylheptyloxy)-1,4-phenylene) (CNPPP) were shown to be potentially tunable across the range of 1000 to 1600 nm. The EL intensity of the PbS NCs

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Figure 24. a) The PL spectrum of InAs/ZnS NCs in solution (dashed line) and the EL spectrum of an InAs/ZnS NCs/MEH-PPV LED (& and solid line). b) Current (dashed line), light (solid line), and external quantum efficiency (solid line, right axis) of the LED as a function of applied voltage. Reprinted with permission from Ref. [139]. Copyright 2002, AAAS.

capped with octylamine (eight carbon atoms in the chain) was much higher than those capped with oleic acid (18 carbon atoms in the chain), which was explained by the suppression of either Fçrster energy transfer or direct carrier transfer from the polymer to the NCs in the case of the longer ligands. Single-layer devices based on a blend of methyl-substituted ladder-type poly(para-phenylene) (MeLPPP) and HgTe NCs synthesized in water and subsequently transferred to toluene were fabricated by spin-coating from the toluene solution and sandwiching between indium tin oxide (ITO) and Al electrodes.[141] The emission wavelength of the NCs was tunable between 900 and 2000 nm by changing the NC size. These have shown both blue–green emission from MeLPPP and an IR emission from the HgTe NCs, the latter with a turn-on voltage of 10 V and an external quantum efficiency of roughly 0.001 % for an unoptimized device. Following on from the developments of trilayer hybrid visible NC-based LEDs,[142] trilayer NC/organic-molecule near-infrared (NIR) LEDs with a single monolayer of PbSe NCs sandwiched between two organic thin films were presented.[143] The fabrication technique employed was similar to the visible CdSe/ZnS-based LEDs[142] and utilized the self-segregation process of the PbSe NCs (capped by oleic acid) from the hole-transporting organic molecules N,N’-di-

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phenyl-N,N’-bis(3-methylphenyl)-(1-1’-biphenyl)-4,4’-diamine (TPD) or 4,4-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (a-NPD) in chloroform. An electron-transporting layer of aluminum tris(8-hydroxyquinoline) (Alq3) and/or bathocuproine was thermally evaporated on top. The IR EL spectra of the devices so fabricated were tunable from 1.33 to 1.56 mm by changing the size of the PbSe NCs from 4 to 5 nm, with a fwhm of < 160 nm (< 0.11 eV). The external quantum efficiency of the NIR EL of these devices was measured to be 0.001 %, which was limited by the reduced PL quantum efficiency of closely packed PbSe NCs in the solid state as well as the unoptimized spectral overlap of the organic components and IR-emitting NCs.

3.3. Near-Infrared-Emitting Nanocrystals for Biological Imaging The prospects of using semiconductor NCs as fluorescence labeling reagents for biomedical assays and imaging experiments are treated enthusiastically in the research community, and hundreds of papers have appeared since the introduction of this idea in 1998.[144] Size tunability of the emission spectra of NCs for one and the same semiconductor material, in combination with a rich surface chemistry allowing the use of different bioconjugation strategies, make them exceptionally versatile probes for fluorescent labeling experiments. Much effort has been put into rendering hydrophobic CdSe-based NCs synthesized in nonpolar organic solvents water soluble,[145, 146] while the brightly emitting CdTe NCs synthesized directly in water are also available.[147, 148] Semiconductor NCs generally show improved stability towards photobleaching in comparison to commonly used organic chromophores, and their absorption cross sections are one-to-two orders of magnitude larger then those of organic dyes.[149, 150] Furthermore, NCs have nearly continuous excitation spectra above the threshold of absorption together with a strong, narrow, and symmetric emission band, in contrast to the typically broader and asymmetric emission profiles of organic dyes. These properties are particularly useful when many-color probes need to be simultaneously excited by a single narrow-band excitation source and distinguished in a single exposure. The long fluorescence lifetimes of NCs, typically on the order of several tens of nanoseconds, are advantageous for distinguishing them from background autofluorescence in cells. Optical in vivo imaging in biomedicine requires the use of excitation and emission wavelengths where absorption and scattering by biological fluids and tissue (water, hemoglobin, lipids) are as low as possible.[151] The near-infrared spectral region (the so-called biological window between 700 and 900 nm) of emission wavelength satisfies these requirements, providing deeper penetration of photons from the excitation source and greater escape depths for the emission signal in a tissue. The potential uses of near-IR emitting NCs as fluorescent contrast agents for biomedical imaging in living tissue is thus exceptionally high, given the very limited choice of suitable organic dyes (Cy7, IRDye78, indocyanine green) that can be utilized within the biological small 2007, 3, No. 4, 536 – 557

window. Both the fluorescence quantum yields of NCs are higher and their photostability is truly superior when compared to the above-mentioned dyes, in addition to the general advantages of NCs presented above. Near-infrared emission wavelengths of NCs used as contrast agents in both cells and tissue are additionally advantageous because of the greatly reduced autofluorescence background signal. Following mathematical modeling of the effects of tissue absorbance and scattering as a function of water-to-hemoglobin ratio on the performance of IR NCs as fluorescent contrast agents,[4] Bawendi and co-workers have successfully demonstrated virtually background-free sentinel lymph node mapping of living mice and pigs by means of the intradermal injection of near-IR type II CdTe/CdSe NCs emitting at 850 nm.[152] This was followed by the use of InAs/ZnSe NCs with a hydrodynamic diameter < 10 nm (smaller than the typically used core/shell CdSe-based NCs) for imaging sequential lymph nodes in rats.[51] CdHgTe NCs (additionally doped with Mn2 + ions showing strong EPR signals) were used as a contrast agent for vessels surrounding and penetrating a murine squamous cell carcinoma in a mouse.[153] The latter study additionally stressed an exceptionally high potential of near-IR NCs as contrast agents in pharmacokinetic analysis.

4. Summary and Outlook The fundamental possibilities to study infrared-emitting colloidal NCs and their application potential are wide in scope. The spectral range across which these materials have the potential to be applied is precisely the range of frequencies that the organic-molecular-based architectures have so far had limited success in addressing. There are, for example, no organic dyes that have demonstrated photostabilities on the timescales required by biological applications in the NIR. The fact that the IR NCs are tunable across exactly this range of frequencies opens up the possibility to extend the spectral range applicable to biological labeling and multiplexing. Aqueous-based systems possessing sufficiently high quantum yields are a prerequisite here and the aqueous synthesis of thiol-capped II–VI IR NCs, which has recently being modified to give access to emission wavelengths from 1.2 to 3.7 mm,[154] extends the set of materials upon which biological applications may be based.[155] Another important issue connected to the biological applications of IR NCs is the issue of the ligand selection at our disposal. This is presently a fast-moving field and many of the problems associated with ligand removal and replacement are expected to be adequately addressed. The need to increase the volume of telecommunications traffic makes the drive for systems with broad bandwidth a necessity. The incorporation of the IR NC into both glass and polymer matrices has been demonstrated, but improvements in efficiencies are required before the europiumbased systems can be competitively competed with. As mentioned, the all important windows at 1310 nm and 1550 nm are in the main focus of this development but the development of water-free silica fibers, where the hydroxyl absorp-

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tion band at 1400 nm has been greatly diminished, extends the spectral frequency range possibilities that may be presently employed in signal transmission and opens the way for developments using wavelength-division multiplexed architectures. The above facts coupled with the already demonstrated ability of the lead chalcogenides to display amplified emission and multiple excitation generation paves the way for NC-based IR lasers and higher-efficiency third-generation solar cells. The gain in solar-cell efficiencies due to the conversion of a high-energy photon to multiple lower energy photons can be achieved because the additional contribution to the photocurrent is not accompanied by the extra heat generation that results from phonon scattering. Other applications of the NCs are as IR and, with the appropriate dopants, very-long-wavelength detectors. Indeed the majority of the detectors already in existence for NIR detection are based on lead sulfide and lead selenide and it is hoped that by manipulation of the bandgap the detection wavelength range and sensitivities can be tailored. In fact the potential usefulness of these materials in all applications that require absorption and emission in the red and near-infrared region of the spectrum cannot be overemphasized.

Acknowledgements The authors gratefully acknowledge all the colleagues who have contributed to the work on infrared-emitting colloidal nanocrystals over the years; their names appear in the list of references. Also gratefully acknowledged is the assistance of Andreas Kornowski and Sylvia Bartholdi-Nawrath (University of Hamburg) in attaining the TEM and SEM images that appear in the frontispiece. The subsection on LED applications of infrared nanocrystals forms a section of the chapter on “Hybrid OLEDs with Semiconductor Nanocrystals” from the book Organic Light-Emitting Devices (Eds: K. M-llen & U. Scherf), Wiley-VCH, Weinheim, Germany, 2006.

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