Subscriber access provided by EASTERN KENTUCKY UNIVERSITY
Review
Flat colloidal semiconductor nanoplatelets Cecile Bouet, Mickael D. Tessier, Sandrine Ithurria, Brice Nadal, Benoit Mahler, and Benoit Dubertret Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm303786a • Publication Date (Web): 15 Feb 2013 Downloaded from http://pubs.acs.org on February 18, 2013
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Flat colloidal semiconductor nanoplatelets Cecile Bouet, Mickael Tessier, Sandrine Ithurria, Benoit Mahler, Brice Nadal, Benoit Dubertret* Laboratoire de Physique et d’Etude des Matériaux, CNRS, ESPCI, 10 rue Vauquelin, 75005 Paris, France ABSTRACT: Free standing two-dimensional nanostructures, referred to as nanoplates, nanomembranes or nanosheets are at the center of intense research and development. These structures can be metallic, dielectric, or semiconductor and have a controlled and a uniform thickness that is small compared to their lateral dimensions. Some of these structures are grown in solution, and emerge as a novel class of colloidal material with physical properties close to the quantum wells, and a chemistry similar to the one developed for the colloidal quantum dots. They can be easily manipulated, and their surface chemistry can be tuned with methods similar to the ones developed for the colloidal nanocrystals. Here, we review the syntheses in solution, the properties, and the first applications of these flat colloidal semiconductor nanostructures.
INTRODUCTION Two-dimensional (2D) semiconductor crystals with a thickness much smaller than their lateral dimensions are one of the key elements of modern microelectronic and optoelectronic. These 2D structures have thicknesses that can range from few microns, to just few atomic layers, and lateral dimensions that can reach few centimeters. Amongst these structures, the ultrathin ones, known as quantum wells have been studied in the early 1970’s, with a strong motivation to develop new types of devices1 such as the Bloch oscillator, or quantum well lasers. With the advent of a new growth technique, the molecular beam epitaxy2 (MBE), semiconductors could be grown atomic layer upon atomic layer, and in 1974, two basic experiments reported the quantum behavior of the charge carriers3 and the quantization of energy levels in quantum wells4, as predicted in elementary example of quantization in quantum mechanics textbooks5. Since then, studies of ultrathin semiconductor layers have proliferated at an explosive rate and various industrial applications such as lasers and infrared detectors have emerged. The ultrathin semiconductor layers grown by MBE, or other deposition methods, are usually grown on a thick, solid, crystalline substrate that serves both as a seeding layer for the crystal growth, and help withstand the high temperature and vacuum used during the growth processes. This substrate may be interesting in several aspects, but it can limits the processability of the ultrathin semiconductor layers, especially when assembly, or orientation control is desired. Recently, ultrathin semiconductor layers have been obtained or synthesized in a free-standing6 form, so that the ultrathin layers can be manipulated without their substrate. The advantage of free-standing 2D semiconductor structures is that they can be manipulated using transfer processes7, assembled with Langmuir Blodgett films8 or self-assembled9. They also have unique mechanical properties, one of the most important being their flexibility and their ability to roll, bend, and adapt
their shape to a deformable substrate10. The backside is that they may aggregate easily in solution, their surface may be more easily contaminated, and the production of more than few monolayers thick structures, and heterostructures has been demonstrated only recently, so that the variety of materials that were demonstrated to form 2D colloidal structures is much more limited than in the case of quantum wells synthesized with epitaxial methods. Free-standing structures with 2D geometry can be obtained using various techniques that we now briefly discuss. 2D-free-standing structures can be obtained using topdown approaches. They can be exfoliated mechanically, as in the case of graphene11 with a tape in air or in vacuum to yield materials with a surface devoid of contaminating organic ligands or molecules. When larger amount of material is needed, they can be exfoliated in liquid12. Exfoliation in liquid has been demonstrated mainly with layered compounds, but it has recently been extended to non layered compounds such as ZnSe13. They can also be produced using a strategy that takes advantage of the large experience of the epitaxial growth world14. In this case, the ultrathin film is grown with MBE or Molecular Organic Chemical Vapor Deposition (MOCVD), and then pealed off from its substrate15,16, an approach similar to exfoliation. Recent strategies have emerged making use of a sacrificial layer that is deposited between the substrate and the 2D structure of interest17. Once the sacrificial layer is removed, usually dissolved, the 2D structure becomes free floating and can be manipulated. The most widely used method for the production of freestanding 2D-structures is to synthesize them directly in solution. The solution growth approach has undergone an extremely rapid development in the last few years. The first attempts of 2D synthesis in solution were performed at the air/liquid interface18 with ligands selfassembling at the interface so that a flat surface was formed and could serve as a template for the growth of
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
crystalline 2D structure. This technique is still developed for the synthesis of new nanoplates such as PbS19, and it has been revisited recently for the formation of large zinc hydroxide nanomembranes at the water-air interface in presence of sodium dodecyl sulfate9. Soon after the first syntheses at the liquid interface, it was realized that molecular precursors self-assembling in solution could form a lamellar structure serving as a mold for the synthesis of flat, layered structures20,21. The structures obtained with this technique are usually one or two monolayers thick. Solvothermal methods have also been developed for the synthesis of free-standing 2D structures.22 In this case, the syntheses are performed in an autoclave under high vapor pressure and high temperature (usually slightly above the solvent boiling temperature), in the presence of a stabilizing molecule (most of the time poly(vinyl pyrrolidone)). Various semiconductor materials have been synthesized with these techniques23-25, that yield 2D structures with lateral dimensions that can reach few microns. The latest techniques developed for the solution synthesis of 2D free-floating structures are extensions of the methods already developed for the synthesis of colloidal semiconductor particles. The origin of this field can be traced back to the exploratory work of Ekimov26, Henglein27, Efros28 and Brus29 with spherical, cadmium based, compounds, in the early eighties. In this approach, the control of the nanoparticle shape is not imposed by a substrate, or molded at an interface, but rather, it results from a subtle interplay between the ligands and the different facets of the nanoparticles that are grown in solution30. Ligands that bind strongly to specific facets prevent, or slow down their growth, while other facets with weakly bound ligands can grow fast. With a good choice of ligands, nanoparticles with various shapes from spheres, to nanorods31, nanowires32, tetrapods33, and now 2D structures34 can be synthesized (Figure 1). One of the reasons for the growing interest in the synthesis of 2D structures using the colloidal methods is that there has been a very large amount of work performed to understand in details the synthetic mechanisms and the growth dynamics of 0D and 1D colloidal semiconductor structures, to tailor their composition to obtain for example core/shell structures, to assemble them and to connect them, and it is hoped that some of this knowledge can help for the development of high quality colloidal 2D structures.
Page 2 of 13
Figure 1: Colloidal CdSe nanocrystals with different shapes. Left, spherical nanoparticles with 3D confinement. Middle, CdSe quantum rods with 2D confinement. Right, CdSe nanoplatelets with 1D confinement. The synthetic approaches we have detailed have been used for the synthesis of colloidal 2D structures of various materials such as oxide35,36, dielectric37, metals38, and the reader is refered to the very good reviews already written on these materials for further information. In this review, we will focus only on the 2D colloidal structures made of semiconductors. We will further restrict our discussion on the colloidal structures that are grown in solution with ligands that bind to the nanoparticles and help define their shape. As mentioned above, these 2D colloidal structures are an extension (in terms of shape) of colloidal quantum dots and nanorods or nanowires obtained with similar synthetic approaches. Indium based colloidal nanoplatelets In2S3 is an n-type semiconductor with a band gap of 2.02.3eV39. It exists in three different crystal structures: αIn2S3 (defect cubic), β-In2S3 (defect spinnel), γ- In2S3 (layered structure)40. It has been integrated as a phosphor in displays, and as a buffer layer in solar cell devices with conversion efficiencies larger than 12%, similar to the ones obtained with the standard CdS buffer layer41,42. Colloidal indium sulfide β-In2S3 nanoplatelets (NPLs) have been synthesized starting from 200643. The motivation of the work was to develop an alternative technique for the growth of In2S3 films using self-assembly of In2S3 NPLs. The authors report the synthesis of monodisperse β-In2S3 hexagonal NPLs that are 0.76 nm thick and have tuneable lateral dimensions from 22 nm to 63 nm (figure 2). The synthesis is performed by injection of anhydrous InCl3 and sulfur powder dissolved in oleylamine, in oleylamine heated at 215 °C. After 1 hour, a bright yellow precipitate is formed with In2S3 NPLs as the sole product of the reaction when indium is in large excess compared to sulfur. The In2S3 NPLs are characterized with Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and absorption spectroscopy. Interestingly, In2S3 NPLs absorption spectrum display a step-like structure that has been interpreted as a transition between the conduction and the valence band of In2S3. Self-assembly of the NPLs is demonstrated with TEM with stacks of different geometries, depending on the alignment, parallel or upright, of the NPLs vs the substrate. Very recently, the same team proposed to use columnar assembly of In2S3 NPLs heated under argon and deposited on graphene as new anode material for lithium ion batteries44. When the assembly of In2S3 NPLs is heated at 400 °C under argon, the columnar structure remains while its carbon/hydrogen ratio shifts from 6 to 48. This increase is interpreted as a transformation of the original NPLs ligands into carbon material relocated between the NPLs. The heat-treated graphene- In2S3 composites show enhanced discharge capacities, up to 716-837 mA.h.g-1, as well as excellent stability. Similar results have been obtained with In2S3 thin films synthesized in solution45 and seem to be extendable to other materials. In2S3 NPLs have also been synthesized with larger dimensions, up to few micrometers, using indium-tris
ACS Paragon Plus Environment
Page 3 of 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
diethyldithiocarbamate mixed with oleylamine and hexadecylamine, and heating to 200 °C for 30 min and then to 300 °C for 2 h46. After doping these NPLs with copper ions, it was suggested that doped and undoped particles presented similar photoresponse and conductivity properties. In addition, In2S3 micronsized nanoribbons have also been synthesized using a solvothermal route47 and studied for their photocatalytic properties.
Figure 2: TEM images of a) 63 nm b) 45 nm, and c) 33 nm In2S3 NPLs, d) side view of NPLs. e)- h) particle size distribution. Reproduced with permission from ref. 43 Copyright 2006 Wiley-VCH Verlag GmbH & Co.
Tin based colloidal nanoplatelets Four types of tin colloidal nanoplalets have been synthesized so far: SnSe248, SnS249, SnS49, and SnSe50. The first two materials are members of the metal chalcogenide materials MX2, with a layered structure such that the plans of metal atoms are sandwiched between two planes of chalcogenides. These materials have two interesting attributes: two successive layers can be readily separated, and host molecules can be easily intercalated, as first shown by Dines in 197551. The tin-based chalcogenides crystallize in hexagonal close-packed CdI2-type structure, and have a symmetry described by the D33d space group. Both SnSe2 and SnS2 have indirect bandgap52, and have been foreseen as interesting candidates for cathodes materials in Li-batteries or for photovoltaic applications. SnSe2 NPLs have been synthesized with an injection of 1,3-Dimethylimidazoline-2-selenone dissolved in a mixture of oleylamine and dichloromethane in a solution of SnCl2 dissolved in dried oleylamine at 220 °C. After a reaction time of 2 h, the NPLs were precipitated, mixed with graphene oxide and hydrazine monohydrate, and heated at 80 °C, which resulted in mixed composite of SnSe2 and graphene oxide. The resulting material was applied as anode materials for lithium batteries and showed promising storage performance, superior to SnSe2 NPLs alone on graphene. SnS2 NPLs with a roughly circular shape, lateral dimensions of 150 nm and a thickness of 6 nm have been synthesized using a protocol similar to the one used for the synthesis of SnS NPLs described below, modified to include oleic acid49. Tin sulfide SnS is one of the four layered semiconductor (SnSe, GeS and GeSe are the three others) which form an isomeric subset of the IV-VI materials. These materials
have an orthorhombic crystal structure53, close to a distorded NaCl structure. This type of structures is particularly suitable for the synthesis of 2D colloidal NPLs since their cationic layers are arranged in the crystal structure such that they are held together only through Van der Waals forces, which provides a chemically inert surface devoid of dangling bonds and a low density of surface states54. This results in materials with very good chemical and environmental stability55. SnS has both a direct (1.09 eV) and an indirect (1.3eV) bandgap56. It has also a large absorption coefficient (~104cm-1) and has been perceived as a good active absorber in solar cells. Orthorhombic very large SnS NPLs have been synthesized in oleylamine with a single source precursor made of tindiethyldithiocarbamate, heated for 1 h at 300 °C. The NPLs obtained are rectangular with very large dimensions that could reach 7 µm x 3 µm x 20 nm, with a rather homogeneous thickness on the whole NPL surface. These ultra large NPLs were tested as anode material in a Li-ion battery. The galvanostatic test showed that the SnS NPLs exhibited a flat voltage plateau with a capacity of 350 mA h g-1 around 1.2 V versus lithium metal in the initial discharge step. The capacity of the SnS NPLs slowly fell down after few charge/discharge cycles49. To end the colloidal tin based NPLs series comes the SnSe NPLs. SnSe is a narrow bandgap semiconductor with a predicted indirect bandgap of 0.9eV and a direct band gap of 1.3 eV57. It also has a large absorption coefficient and is of interest as a light absorption layer in solution processed solar cells and near-infrared optoelectronic devices58. Colloidal SnSe nanoparticles have only recently been synthesized59,60 and colloidal SnSe NPLs were synthesized last year using a one-pot synthesis with a slow heating from room temperature to 240 °C of a mixture of tin chloride, oleylamine, TOP-Se and hexamethyldisilazane50. The resulting NPLs with average dimensions on the order of 500 nm x 500 nm have a thickness that ranges from ~8 nm to ~35 nm depending on the reagent concentration (figure 3). The SnSe NPLs formation pathway was investigated using TEM snapshots of the growth solution at various times. In the earliest stages, individual nanoparticle seeds are observed along with small aggregates that appear to grow and coalesce into dendritic 2D structures that eventually evolve into uniform square-like nanosheet morphology. Further evolution with a growth in the thickness direction can be achieved possibly through the oriented attachment of nanoparticles on the NPLs surface. This study provides an analysis of the possible growth mechanisms of 2D objects in solution, and is one of the few exemples to date of NPL thickness control. SnSe NPLs have also been shown to react with trioctylphosphine –tellurium complex and to transform into SnTe porous NPLs 61, and single layer SnSe nanosheets have been synthesized and films of SnSe nanosheets used as photodetectors62.
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 13
toluene solution) as the upper oil phase, which plays the role of sulfur source, ligand, and reducer. The lower aqueous phase contains copper ions and added anions used as ligands for adjusting the growth of Cu2S. The synthesis was performed in an autoclave at 200 °C and produced highly uniform Cu2S nanocrystals with dimensions ranging from 8nm to 13nm, thicknesses of ~5 nm, that can self assemble into highly ordered multilayer superlattices, with the possibility to adjust the packing symmetry of the superlattice. Other synthesis methods7173 for Cu S NPLs have been reported, with often a very 2 good shape and size uniformity leading to an easy selfassembly. Copper selenide cubic berzelianite NPLs 74 with lateral dimensions superior to the micrometer have also been synthesized using copper chloride and selenium powder mixed in oleylamine, 2-ethylhexanoic acid and paraffin, and heated at 250 °C. Smaller Cu2-xSe NPLs with lateral dimensions close to 17nm and thickness of 2.6nm have also been synthesized with 1,3dimethylimidazoline-2-selenone as a selenium source. These NPLs could also assemble and their optoelectronic properties were studied75.
Figure 3: (a) TEM image of SnSe NPLs with corresponding size distribution histogram (inset), (b) higher magnification image. Adapted with permission from ref. 50. Copyright 2012 American Chemical Society. Copper based nanoplatelets Copper sulfide nanodisks with two different compositions were first synthesized by two different approaches: a solventless synthesis63,64 and a solution based arrested precipitation65. The solventless synthesis yields nanodisks of chalcocite Cu2S with a hexagonal crystal structure. Cu2S is a p-type semiconductor with a bulk band gap of 1.2 eV and CdS/Cu2S has been studied in the 1960s and 1980s as very prominent thin film photovoltaic cells with conversion efficiencies larger than 9%66. This system has recently been revisited with solar cells based on Cu2S nanoparticles mixed with CdS nanorods67, or multiwalled carbon nanotubes68. The arrested precipitation in a hot solvent produces under certain reaction conditions nanodisks of covelite CuS, which has also a hexagonal structure. Both methods provide relatively size- and shape-monodisperse nanodisks with diameters ranging from 14 to 20 nm and thicknesses between 5 and 7 nm that could self assemble in columns of ~200 nm69 (figure 4). Cu2S NPLs have also been synthesized in a two-phase reaction system70, with dodecanethiol (or its
Figure 4: TEM images of CuS and Cu2S nanodisks. (A) Single-layered CuS nanodisks lying on their faces. (B) Linear chains of stacked Cu2S nanodisks. (C) CuS nanodisks crystallized into a “T”- shaped structure. (D) Ordered Cu2S nanodisk assembly oriented parallel to the substrate. (E) Illustrations of different nanodisks assemblies and orientations on the substrate: (i) a monolayer; (ii) columnar assembly with columns oriented perpendicular to the substrate; (iii) columnar assembly with columns oriented parallel to the substrate. Reproduced with permission from ref. 69. Copyright 2006 American Chemical Society. Lead based nanoplatelets
ACS Paragon Plus Environment
Page 5 of 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Ultrathin single PbS crystal sheets with dimensions on the micrometer scale (figure 5) have been synthesized using the oriented attachment of lead sulfide nanocrystals76. The oriented attachment of the PbS nanocrystals was obtained thanks to the addition of a chlorine containing co-solvent during the synthesis. The authors conjectured that the oriented attachment of the PbS nanocrystals lead to 2D structures rather than 3D structures because the oleic acid assembles in a highly ordered monolayer on the [100] surfaces of the nanocrystals and plays a key role in the formation and further stacking of PbS NPLs. Further studies of the PbS 2D nanocrystal formation under deviatoric stress confirmed the ordering of PbS nanoparticles into single crystal nanosheets with a lamellar structure77. Small-angle x-ray scattering was performed on the stacked PbS NPLs, and the scattering pattern was best fit with a model assuming the regular stacking of disks with a long period of 5.8 nm and PbS sheet thickness of 2.2 nm. The resultant distance between the sheets of 3.6 nm is attributed to an oleic acid bilayer. HRTEM images of the PbS NPLs show single crystalline structure with a growth parallel to the axis. Some holes are present in the NPLs structure as well as some corrugation. The absorption spectra of the NPLs do not show pronounced excitonic structure as it would be expected for NPLs with a very regular thickness and lateral dimensions larger than the exciton Bohr radius. The photoconductivity of these PbS NPLs was measured and yielded promising results in part because of missing in-plane ligands and continuous connection through the evaporated sample. PbS nanostructures of zero-, one- and two- dimensions have also been synthesized in trioctylamine with a low temperature decomposition of a single precursor, lead ethyl of hexadecyl xanthate78. In this case, the evidence of the presence of some two-dimensional structures was obtained with TEM images.
Figure 5: TEM image of stacked PbS nanoplatelets synthesized using 1,1,2 trichloroethane. From76. Reprinted with permission from AAAS. Cadmium based nanoplatelets One of the characteristics of cadmium based nanoparticles, and more generally II-VI semiconductors, is that they have strong oscillator strength and direct band gaps in the visible. It results that many of the nanoparticles morphological properties, such as size, shape, composition, crystal structure, or surface ligands, are accessible using a careful analysis of the nanoparticle spectroscopic properties, such as fluorescence emission, absorption,
photoluminescence excitation, and fluorescence lifetime. For these reasons, a lot of the pioneering work done with colloidal semiconductor nanocristals has been performed with cadmium-based compounds. After the first demonstration of the size-dependent optical properties of CdSe nanoparticles embedded in glass by Ekimov26, Murray and Norris developed the “hot injection” method to obtain the first spherical colloidal semiconductor nanocrystals of CdE (E=Se, S, Te) with a size dispersion close to few percents79. Hines and Guyot-Sionnest demonstrated the first core/shell structure80 with the growth of ZnS on CdSe nanoparticles. Peng et al. illustrated the importance of the ligands to control the morphology with the synthesis of CdSe colloidal nanorods31, and the first synthesis of nanocrystals with a confinement of the charge carriers in 2 directions – instead of 3 in the case of spherical nanocrystals. Further shape control was also demonstrated with CdSe81 tetrapod. The great knowledge accumulated with the synthesis and the physical properties of the cadmium based nanocrystals could often be transferred directly or with some modifications to the synthesis of nanoparticles of other compounds. In the field of colloidal 2D NPLs, cadmium based materials have also played a key role. Wurtzite Cadmium based NPLs. In 2006, T. Hyeon and co-workers reported the synthesis of CdSe nanoribbons34 that were obtained with the reaction of cadmium chloride alkylamine complex and selenocarbamate at 70 °C, a temperature much lower than the temperatures usually used for other type of CdSe nanoparticles. It was latter reported that the wurtzite nanoribbons were obtain thanks to a soft colloidal template method82. The cadmium chloride alkylamine complex forms a lamellar structure in solution that serves as a mold for the formation of the CdSe ribbons. The CdSe nanoribbons have a width of 10-20 nm, a length that can reach few hundreds nanometers and a uniform thickness of 1.4 nm. These nanoribbons are fluorescent and have an emission spectrum with a full width at half maximum (FWHM) close to 11 nm, much smaller than the CdSe nanoparticles with other shapes. Such narrow FWHM was interpreted as a sign of “extremely uniform thickness”. Because of their size, these nanoribbons readily aggregate in solution83. Nanoribbons with hexagonal structures grow along the [001] c-axis, their width is along the [1 10] axis, and their thickness is in the [110] direction82. The nanoribbons terrace surfaces are the 110 plane, they are composed of trivalent Cd and Se atoms. These surfaces are passivated with primary amines that can weakly coordinate trivalent Cd atoms. Slight modifications of the nanoribbons synthesis yielded CdSe nanosheets82, that are nanoribbons with a larger width (~100nm) and that can be suspended in solution after the synthesis. The low-temperature growth strategy used in the synthesis developed by T. Hyeon and co-workers has been adapted with success to incorporate a large quantity of Mn2+ dopants in CdSe nanoribbons. The high doping level, up to 10%, resulted in a giant Zeeman splitting of the CdSe excitons as large as 54.6 meV with an effective g factor of ~60084. It was also adapted to the synthesis of CdTe nanosheets with hexagonal wurtzite structure using the reaction between the cadmium chlo-
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ride-alkylamine lamellar complex and bis(tertbutyldimethylsilyl)telluride82. The CdTe nanosheets were triangular with a width of several hundreds of nanometers and a thickness of ~1 nm. CdS wurtzite NPLs have also been synthesized with two different thicknesses and with lateral extension ranging from few nanometers to few hundreds of nanometers85. In parallel to the work performed on the nanoribbons, W. Burho and co-workers have synthesized CdSe quantum belts86, from cadmium acetate and selenourea, using slight variations of a procedure first developed by Peng and co-workers87. These quantum belts are structurally and morphologically similar to the nanoribbons. Their first two electronic transitions were analyzed as light hole and heavy hole transition as it has been proposed earlier for zinc-blende NPLs 88. The quantum belts have a quantum yield of up to 30%, much higher than the 0.10.3% obtained for the CdSe quantum wires of similar dimensions89. The authors analyse the PL luminescence difference between the two structures with a higher density of trap sites at the surface of the quantum wires compared to the quantum belts. The formation mechanism of the CdSe quantum belts proceeds through the double-lamellar assembly of (CdSe)13 small clusters90. After their formation, the bundled quantum belts could be unbundled through sonication. The (CdSe)13 clusters had previously been identified as intermediate products in the formation of CdSe nanoribbons by T. Hyeon and co-workers84. It thus appears that quantum belts and nanoribbons are very similar not only in their morphology, but also in their formation mechanism. A thorough review of nanoribbons is provided by T. Hyeon and coworkers in this special issue, and the reader is invited to refer to it for more details. Zinc blende Cadmium based NPLs. Zinc blende CdSe NPLs were synthesized in 2008 by Ithurria88. The NPLs synthesis is based on a variation of the protocol developed by Cao and co-workers91 for the synthesis of zinc-blende spherical nanocrystals. In brief, cadmium myristate is mixed with a powder of selenium in octadecene and heated at 240 °C. When the temperature reaches ~180 °C, an acetate salt is introduced in the solution and CdSe NPLs are formed. The advantage of the zinc blende NPLs is that their lateral dimensions can be tuned from few nanometers to few hundreds of nanometers. When their lateral dimensions are below 100 nm, there are colloidally stable. The first report of the zinc blende NPLs synthesis in 2008 introduced several important features. The first is that TEM images of NPLs clearly established the 2D structure of individual NPLs, with different shapes, depending on the synthetic conditions. The second is that NPLs with three different thicknesses were synthesized in thickness-pure populations (figure 6). This was, to the best of our knowledge, the first report of colloidal 2D objects with tunable thickness controlled at the atomic level. The third is that the onedimensional confinement of the exciton was clearly established. The electronic transitions of the NPLs were compared to the ones of the epitaxial quantum wells, and the first two electronic transitions observed in the absorption spectra were assigned to the light hole-electron
Page 6 of 13
and the heavy hole-electron transitions. Fourth, the absorption and the emission spectra of pure populations with the three different thicknesses showed the smallest FWHM reported so far, less than 8 nm, that is ~1.2 kT, for the three populations. This analysis of the NPLs emission and absorption spectra using a crude model of a quantum well with an infinite barrier showed that the thickness differences between NPLs were exactly integer multiples of one CdSe monolayer. It was concluded that the zinc blende NPLs were atomically flat. This model neglected the exciton binding energy, and could not predict correctly the exact NPLs thickness. In contrast with the wurtzite CdSe 2D structures, the zinc blende NPLs have their thickness in the [001] direction and the two other principal crystallographic axes form the lateral NPLs plane. The top and bottom faces of the NPLs are pure cadmium plans passivated with carboxylate ligands. The analysis of the growth mechanism of the CdSe NPLs showed that they could be lateraly extended through a continuous reaction of precursors92. We did not observed oriented attachment nor the formation of small CdSe clusters during the lateral extension of the NPLs. In addition, the lateral extension of CdSe NPLs can be performed at temperatures as high as 260 °C, a temperature at which the organic ligands lamellar assembly is not stable. This suggests that in these conditions, the growth mechanism of zincblende NPLs is radically different than the one observed for the wurtzite CdSe nanoribbons and nanowires. Since the NPLs lateral extension could be tuned from few nanometers to several tens of nanometers, we have been able to monitor the spectroscopic evolution of the emission and the absorption spectra vs the lateral size and thus the evolution of the exciton confinement from a 3D confined system (a sphere) to a purely 1D confined system (a nanoplatelet)92. The synthesis of CdSe zinc blende NPLs was studied in more details by Peng and co-workers93. They explored the influence of different carboxylate alkyl chain length on the morphology of the NPLs, as well as various temperature conditions for the CdSe NPLs growth. The synthesis of CdSe NPLs was rapidly extended to CdTe and CdS NPLs with different thicknesses94. The synthesis of CdTe NPLs is similar to that of CdSe except that the anion injection is done at higher temperature to ensure the formation of thicker NPLs. During the CdTe NPLs growth, the thinnest NPLs disappear whereas thick NPLs gradually form. The CdS synthesis is carried out with a protocol similar to the CdSe synthesis. Populations of CdS NPLs with four different thicknesses could be obtained. In parallel to the CdS syntheses developed by Ithurria, Peng and co-workers reported the synthesis of CdS NPLs optimized so that homogeneous NPLs populations of 4 different thicknesses, depending on the synthetic conditions, could be obtained95. The dependence of the optical transition energies of the CdSe, CdS and CdTe zincblende NPLs has been described quantitatively with a multiband effective-mass approximation94. This eight-band kp model, which simultaneously takes into account the non-parabolicity of the conduction and valence band dispersion, describes very well the size dependence of the optical transitions of the ZB NPLs. The good agreement of the model and the experimental data confirmed that the ZB NPLs are atomically flat, very deep colloidal quantum wells, whose properties are con-
ACS Paragon Plus Environment
Page 7 of 13
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
trolled solely by the NPL thickness. Due to the dielectric confinement effect and the very large confinement energy enhanced through the mirror charges effect96, this class of free-standing colloidal quantum wells may have significantly different optical and electrical properties from spherical nanocrystals. They may combine two interesting characteristics of both worlds: the wide tunability of the absorption and photoluminescence of NCs and the short decay time of excitons in quantum wells. Indeed, sub-nanosecond radiative fluorescence lifetimes where observed in CdSe NPLs at cryogenic temperatures94. These lifetimes are two orders of magnitude shorter than the ones observed on spherical CdSe nanoparticles. This makes the NPLs the fastest colloidal fluorescent emitters to date and suggests that they show a giant oscillator strength transition97. The fluorescence lifetime of spherical CdSe nanocrystals (NCs) with 10% quantum yield was compared to the fluorescence lifetime of CdSe NPLs with 40% quantum yield. Even in this unfavorable situation, NPLs have much faster lifetimes than NCs, suggesting that the binding energy of the exciton in the NPLs is much greater than the one in NCs. Transient absorption measurements of carrier relaxation after an intense excitation with a laser pulse showed that the high temperature carrier population cools back to room temperature within few picoseconds, which confirms further that the CdSe zinc blend NPLs behave similarly to quantum wells98. The spectroscopy of single CdSe NPLs confirmed the results obtained in ensemble measurements and revealed several interesting features99. The first one is that at room temperature, the FWHM of a single NPL is identical to the one obtained on ensemble measurement. There is thus no inhomogeneous broadening in these materials. The second is that the lifetime of a single NPL at room temperature is similar to the one measured in ensemble. This shows that the highly multiexponential fluorescence decay observed in ensemble results from multiple decay channels in a single NPL and not from a mixture of NPLs with different fluorescence lifetimes. The third important feature is that at cryogenic temperatures, the lifetime of a single NPL is monoexponential and close to 300 ps, again similar to the value measured in ensemble. As for the FWHM, at 20 K it is resolution-limited and is lower than 0.4 meV.
Figure 6: CdSe NPLs. a) absorption (black) and emission spectra (color) of CdSe NPLs with different thicknesses labeled with the number of monolayers of each NPL
population. b) and c) TEM pictures of 5ML and 6ML CdSe NPLs. Adapted with permission from ref 94.
The CdSe colloidal NPLs in wurtzite or zinc blende crystal structures have terrace surfaces that are extremely well defined in terms of composition. As a consequence, they provide a model system to study the growth, the ligand affinity, and the photochemical activity of different crystal planes. For the wurtzite NPLs, the terrace surfaces are the (110)W plane, composed of trivalent Cd and Se atoms, passivated by amine ligands. In the zinc blende NPLs, the terrace are the (001)ZB planes formed of divalent Cd atoms, and are passivated by carboxylic ligands. Lim et al. have studied the photochemical etching of these two types of NPLs, and they have established a relationship between the nanocrystal surface and the photochemical activity of the exciton100. The rate of etching in chloroform is much faster for wurtzite NPLs than for zinc blende NPLs, with the rapid appearance of small holes in the wurtzite NPLs while the ZB NPLs are slowly eroded on the corners and the edges. The authors conclude that the amine (110)W planes are photochemically active with the possible extraction of exciton through these surfaces. On the opposite, the carboxylatepassivated surface of the divalent Cd (001)ZB planes are not.
Other types of nanoplatelets FeS2 NPLs 101 were synthesized using a thermal decomposition of Fe(CO)5 in an organic solvent. Elemental sulfur is dissolved in oleylamine, and heated at the reaction temperature that can vary from 120 °C to 240 °C. Hot iron pentacarbonyl is then injected in the sulfur solution and stirred for different aging times (from 3 min to 540 min). The resulting reaction product is isolated and characterized with TEM. When the reaction temperature is fixed at 240 °C NPLs with lateral dimensions from 200 to 500 nm and thickness of 30 nm are obtained. The NPLs shape varies from hexagonal to triangular. The FeS2 NPLs were blend with organic P3HT and formed a hybrid solar cell with a conversion efficiency of 0.03%. Transition Metal Dichalcogenide (TMD) NPLs including all the early transition-metal (group IV and V) chalcogenide (sulfide and selenide) have recently been synthesized via the reaction of metal chloride and carbon disulfide or elemental selenium in the presence of oleylamine102. 2D layered TMD are held together by Van der Waals interactions, and may have interesting physical and catalytic properties connected to the presence of d-electrons in the transition metal103. Other examples of NPLs synthesized by the colloidal route include semiconducting materials GeS, GeSe104, WS2105, ZrS2106, as well as non semiconducting materials LaF3107, Bi2Te3108, Gd2O3109 and GdF3110 (figure 7). The existence of these 2D colloidal materials illustrates the versatility of the colloidal synthesis method for the growth of 2D plates.
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Figure 7: TEM images of GdF3 (a) ellipsoidal, (b) rhombic nanoplates assembled into (c,d) columnar and (e,f) lamellar liquid crystalline superlattices through a liquid interfacial assembly technique. The top inset is a highmagnification TEM image, and the bottom inset is smallangle electron diffraction pattern. Reprinted with permission from ref 110. Copyright 2011 American Chemical Society. Core/shell nanoplatelets We have discussed several examples of colloidal 2D semiconductor structures and more generally of freestanding 2D structures. All these examples deal with NPLs of only one material. Recently, the first two examples of colloidal 2D heterostructures have been reported111,112. Heterostructures comprise two or more different materials joined together, and have unique properties that can be controlled by the composition, the size and the shape of each component of the heterostructure. Colloidal heterostructures such as core/shell nanoparticles have been synthesized for various compounds113,114 with particles that have the shape of spheres or rods. But heterostructures with 2D geometry have mostly been restricted to the field of epitaxially grown semiconductors, where they are known as multiple quantum wells and are used in several applications such as commercial LEDs or quantum cascade lasers. In spite of their advantages, the synthesis of epitaxial colloidal core/shell structures is complex and several difficulties must be overcome. First, the lattice mismatch between the core and the shell induces pressure that can reach up to 4 GPa in spherical CdS/ZnS core/shell NCs115, and large lattice mismatch induces crystal defects that prevent thick shell growth116. Second, the core/shell interface needs to be carefully controlled since its composition has a strong influence on Auger processes117. It may also reduce blinking118 and improve the fluorescence quantum yield. But core/shell interfaces are hard to control because cations119 and anions120 may diffuse during the shell growth. Third, the type of ligands used for
Page 8 of 13
the shell growth can influence the shape of the final nanostructure as in the case of dot in rods121,122. Finally, the crystal structure of the shell can also be tuned by the ligands used during shell growth123,124. In spite of these difficulties, core/shell colloidal 2D structures have recently been synthesized. One example are the dots-inplate CdSe/CdS125. Two other examples of core/shell colloidal 2D structures are the CdSe/CdS111,112 and CdSe/CdZnS NPLs 111. In these two recent works, room temperature methods were developed for the growth of a CdS or a CdZnS shell on CdSe NPLs. In the work of Mahler, a mixture of thioacetamide (TAA) and octylamine, and cadmium oleate were used respectively as a source of sulfur and of cadmium. At room temperature, a mixture of these precursors with CdSe NPLs results in the formation of CdSe/CdS core/shell NPLs, along with secondary nucleation of CdS nanoparticles. After the reaction, the CdS nanoparticles can be easily separated from the CdSe/CdS NPLs using size selective precipitation. This protocol can be extended to the synthesis of CdSe/CdZnS core/shell NPLs when zinc nitrate is mixed with the cadmium source. The core/shell NPLs have quantum yield that can reach 60% and emission spectra with FWHM usually smaller than 20 nm (figure 8). Upon the shell growth, the emission maximum shifts strongly to the red. For example, core CdSe NPLs with an emission maximum at 510 nm, emit above 600 nm even when the CdZnS shell is as thin as 1 nm. This important red shift is much more important than the one observed on core/shell systems of similar composition but with the shape of spheres of rods. The core/shell NPLs have been characterized using high-resolution high-angle annular dark-field transmission electron microscope images. The core/shell structure could easily be visualized thanks to the difference of atomic density between the core and the shell. The exact number of CdSe planes forming the initial NPLs has also been observed. This provides a direct measurement, with atomic precision, of the NPLs thickness. Using a careful analysis of the deformations inside the NPL with Geometrical Phase Analysis evidenced a tetragonal deformation of the CdSe core. In spite of the large lattice mismatch (up to 15%), the very thin NPLs were able to deform sufficiently to adapt to an epitaxial core/shell growth. In the work of Ithurria et al., the growth of CdS on CdSe NPLs is performed by atomic layer deposition in solution112. Ammonium sulfide or potassium sulfide are used as sulfide precursors, while cadmium nitrate is used as cadmium precursor. Alternative injections of these precursors followed by the elimination of excessive precursors produce a layer by layer growth of NPLs in their thickness. The procedure can be performed in polar or non-polar solvent. These 2D core/shell structures are remarkable in the sense that they are the first step toward the growth of multiple quantum wells grown by MBE or MOCVD, that have found several applications in optoelectronics.
ACS Paragon Plus Environment
Page 9 of 13
Chemistry of Materials *
[email protected].
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Funding Sources B.D. thank the ANR for funding
ACKNOWLEDGMENT We thank the members of the QD team at ESPCI for discussions and fruitful interactions.
Figure 8: (a) Absorbance (black), fluorescence (red), and photoluminescence excitation (PLE, gray) spectra of CdSe/CdS core/shell nanoplatelets (b) High angle annular dark field (HAADF) image of the resulting core/shell nanoplatelets. (c) High-resolution HAADF picture. (d) High-resolution HAADF picture of a core/shell nanoplatelet on the side resolving the core/shell structure and showing the five Cd planes of the initial CdSe nanoplatelet. (e) Geometrical phase analysis (GPA) of the nanoplatelet shown in (d) highlighting the stress between the core and the shell. (f) HAADF picture of an annealed core/shell nanoplatelet sample with very smooth surfaces. Reprinted with permission from ref 111. Copyright 2012 American Chemical Society. Conclusions : The field of ultrathin free-standing 2D structures is evolving very rapidly with new results regarding their syntheses, their physical properties and their applications. While several methods exist to synthesize 2D structures as detailed in the introduction, the syntheses in solution developed primarily for the production of quantum dots or nanorods appear as particularly promising. In this approach, the presence of a substrate or any interfaces are not necessary, the syntheses can be performed at low temperature (
