Silicon Surface Modification and Characterization for Emergent

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Silicon Surface Modification and Characterization for Emergent Photovoltaic Applications Based on Energy Transfer Weina Peng,† Sara M. Rupich,† Natis Shafiq,† Yuri N. Gartstein,‡ Anton V. Malko,*,†,‡ and Yves J. Chabal*,† †

Department of Materials Science and Engineering and ‡Department of Physics, University of Texas at Dallas, Richardson, Texas 75080, United States 6.3. Energy Transfer in Nanocrystal Quantum Dot Solids on Silicon 6.4. Radiative Energy Transfer to Silicon Substrates 6.5. Nanocrystal Quantum Dot Sensitization of Nanostructured Silicon Substrates 6.6. Multiexciton Generation and Singlet Exciton Fission in Applications to Silicon Sensitization 7. Electrical Measurements of Quantum-Dot-Modified Silicon Nanostructures 7.1. Direct Photocurrent Measurements 7.2. Time Resolved Photocurrent Measurements 8. Summary and Outlook Author Information Corresponding Authors Author Contributions Author Contributions Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Chemical Modification of Silicon Surfaces: Oxidized vs Oxide-Free Surfaces 2.1. Background 2.2. Formation of H-Terminated Silicon Surfaces 2.3. Stability of H-Terminated Silicon Surfaces 2.4. Structural Modification of Silicon Surfaces 3. Functionalization of Oxide-Free Silicon Surfaces 3.1. Background 3.2. Hydrosilylation of H-Terminated Surfaces 3.2.1. Background 3.2.2. Catalyst-Aided Reactions 3.2.3. Photochemically Induced Reactions 3.2.4. Thermally Activated Reactions 4. Selected Techniques for Characterizing Wet Chemically Functionalized Surfaces 4.1. Infrared Absorption Spectroscopy 4.2. X-ray Photoelectron Spectroscopy 4.3. Capacitance−Voltage and Conductance− Voltage Measurements 4.4. Surface Recombination Velocity Measurements 5. Deposition of Nanocrystal Quantum Dots on Silicon Surfaces 5.1. Grafting of Nanocrystal Quantum Dots to Functionalized Surfaces 5.2. Layer-by-Layer Assembly 5.3. Self-Assembly of Nanocrystal Quantum Dot Monolayers at the Air−Liquid Interface 6. Energy Transfer Mechanisms from Nanocrystal Quantum Dots to Silicon 6.1. Theoretical Considerations 6.2. Determination of NRET Efficiencies from Nanocrystal Quantum Dots to Semiconductor Substrates

© XXXX American Chemical Society

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1. INTRODUCTION Sensitization of crystalline silicon and other semiconductors by means of energy transfer (ET) may facilitate their utilization in thin-film flexible solar cells and is also interesting in the general context of hybrid nanostructures. ET-based hybrid nanostructures constitute a class of systems that can potentially offer a versatile platform for various optoelectronic applications.1−3 In hybrid systems, the optical and electrical functionalities are clearly separated: one component of the system has strong optical absorption while the other possesses good electrical transport. Importantly, the “coupling” of these components is enabled by a near-f ield electromagnetic interaction that leads to the interconversion of excitons and electron−hole pairs. In the photovoltaic (PV) mode of operation, sunlight is initially harvested in the highly absorbing component of the hybrid, creating an exciton. Then energy transfer across the interface (no charge transfer required) enables creation of electron−hole pairs in the high-mobility semiconductor substrate, where charge separation and transport take place. The separation of functionalities in hybrid systems is similar to photosynthesis,4

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environments and, in some cases, can also lead to surfaces with higher structural and chemical uniformity.16−21 The importance of silicon as a substrate is due primarily to its oxide, which is stable and forms an interface with silicon that is much better than any other semiconductor/oxide interface (e.g., germanium/oxide).22 Surface functionalization of silicon has therefore initially focused on functionalizing OH-terminated silicon oxide surfaces, using silanization with molecules such as triethoxysilanes or trichlorosilanes.23,24 There are, however, several issues associated with silicon oxide functionalization. The process strongly depends on the density of initial surface hydroxyl groups, the number of water molecules present, and the temperature.25 Additionally, silane molecules frequently polymerize, thus hindering the formation of a single self-assembled monolayer (SAM), especially for short molecules (e.g., aminopropyltriethoxysilane). Alternatively, phosphonates can be used, as they are not water sensitive and can be attached to native oxide surfaces under ambient conditions,26−29 although the layers are often not well-defined.14,17−19 For both silane and phosphonate layers, the surfaces are not stable in humid environments, particularly in high-pH liquids required for biological applications. Despite these serious issues, the electrical quality of the initial Si/SiO2 interface has motivated the functionalization of oxides with silanes30−33 or phosphonates34 for the fabrication of most chemical sensors. For grafting NQDs onto silicon for PV cell applications, the main requirements are (1) the density of electronic defect states at the Si surface should be low (1011 photon per event) due to the low cross section (i.e., one/number of photons). Photochemical grafting of alkenes has been successfully carried out using a broad range of energies,155−158 including energies both higher and lower than 3.5 eV. Since hydrosilylation is still successful even when the photon energy is less than the energy needed for homolytic bond cleavage, alternate mechanisms were proposed where photoexcitation of substrate charges may generate active sites.155,156,158 In 2001, Stewart and Buriak proposed a wavelength-dependent mechanism where excitons are generated in the substrate, which can then initiate the reaction.159 These two mechanisms are shown in Figure 7. The electron−hole pair mechanism was validated by grafting alkenes on either oxide-free Si surfaces or oxidized surfaces, as illustrated in Figure 8. It was found that hydrosilylation only occurred on H-terminated silicon surfaces, suggesting that the

Figure 4. Schematic representation of the hydrosilylation mechanism using EtAlCl2 to catalyze the organosilicon reaction. Reprinted with permission from ref 40. Copyright 2012 John Wiley & Sons.

Figure 5. Schematic representation of the hydrosilylation mechanism using a peroxide agent to initiate a radical reaction. Reprinted with permission from ref 40. Copyright 2012 John Wiley & Sons.

Boukkerroub et al.146 and Webb et al.147 on porous and flat silicon surfaces. The mechanism is illustrated in Figure 4.144 Another route utilizes radical initiators, such as diacyl peroxides, to functionalize H-terminated Si surfaces, as shown in Figure 5.148 This catalyst causes a self-propagating chain reaction on the surface in which radicals are formed and consumed. A number of reports have examined, quantified, and confirmed the kinetics related to this reaction using first-principle calculations.149,150 Another method is hydrosilylation initiated by hydride abstraction using compounds such as triphenylcarbenium cations (Figure 6).151 In this case, a positively charged silicon surface is created after the cation removes H−, which in turn enables a nucleophilic attack by the CC double bond (electron-rich). This results in Si−C bond formation and

Figure 7. Schematic illustration of the two pathways for photochemically induced reactions for hydrosilylation of H-terminated Si surfaces: (top) radical-based mechanism and (bottom) exciton-based mechanism. Reprinted with permission from ref 40. Copyright 2012 John Wiley & Sons.

Figure 6. Schematic representation of the hydrosilylation mechanism using a carbocation to initiate an ionic reaction. Reprinted with permission from ref 40. Copyright 2012 John Wiley & Sons. E

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surfaces with minimal interfacial oxide and high photoluminescence (PL) intensities (similar to H-terminated Si surfaces) can be obtained.171−173 The alkyl-SAM-passivated surfaces are more stable than H-terminated silicon, even though the alkene molecules replace 1012/cm2) than those of electrical methods, hence emphasizing the importance of this ALD-based GV characterization method. The method is also sensitive to bulk defect states within the SAM layer, not just those distributed at the SAM/Si interface. In conventional MOS theories,207 the conductance represents the energy loss due to capture and emission of carriers by interface traps. When the surface is populated with a large number of majority carriers, the carriers can move in and out of the defect states so swiftly that no energy loss occurs, i.e., no conductance peak can be observed when the surface is under accumulation. However, for the carboxy-terminated SAM prepared in a singlestep reaction with neat undecylenic acid,166 two conductance peaks are observed, with one in the depletion region and another one in the accumulation region, as shown in Figure 13a,b. The appearance of the second peak is associated with slow states, located further from the interface, similar to border traps present in Si214 and III−V semiconductor junctions.215 This peak is most likely associated with bulk defects within the SAM, because the carrier exchange efficiency between border traps and the semiconductor decays exponentially with distance; i.e., most of these states reside within 1 nm from the interface. Moreover, these states are strongly influenced by an external electric field (Figure 13c): when subjected to repeated scans (i.e., under field influence), the signal from these states gradually increases. The characterization of these states can be carried out through CV measurements.216

The surface recombination velocity (SRV or S) is measured in units of cm/s and is the ratio between the surface recombination rate (Rs) and the surface excess carrier concentration (Δns), as given in eq 2: S=

Rs Δns

(2)

When there is no strong band-bending at the surface, S can be simply expressed as eq 3: S = σvthNit

(3)

where σ is the trap cross section, vth is the thermal velocity, and Nit is the surface trap density (cm−2). Since the total recombination lifetime is determined according to the relationship given in eq 4

τ=

τb

−1

1 + τs−1

(4)

(the subscript b stands for the bulk), the entire recombination process is dominated by the surface term if highly resistive samples (several kΩ·cm) are used. In the past, the interface defect density on H-terminated Si surfaces was determined from SRV measurements, as demonstrated by Yablonovitch et al. in their pioneering work.39 Compared to CV measurements, contactless SRV measurements do not require the formation of metal contacts and therefore do not damage the SAM/Si interface. It was discovered that the alkyl-chain-terminated Si(111) surface,218 prepared by Grignard reactions on a chlorinated Si surface, led to ultralow SRVs. These low velocities are comparable to those measured on H-terminated Si surfaces in contact with concentrated H2SO4, indicating a low defect density. However, this technique is only semiquantitative, and the interpretation of SRV measurements is complicated by the fact that S also depends on band-bending. When strong band-bending exists, such as in the situation of accumulation or inversion, a low surface recombination velocity can be obtained, independent of defect density.219 I

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5. DEPOSITION OF NANOCRYSTAL QUANTUM DOTS ON SILICON SURFACES When considering hybrid NQD/silicon structures, control of the interface and NQD layer is essential. For an ET-based hybrid NQD/silicon structure, the silicon layer is utilized for charge transport while the NQDs layer acts as the absorber. This layer is not composed of single NQDs, but rather macroscopic arrays of NQDs. To improve light absorption, hierarchical films in which each layer absorb a different wavelength of light are desired.220−222 This can be achieved through the fabrication of band-gap-graded NQD films. In this type of structure, excitons are funneled through the band-gap-graded film into the silicon wafer, allowing efficient ET to the substrate to be realized. To fabricate these complex structures, precise control over the deposition of NQDs is essential. Numerous methods exist to deposit NQDs films on various substrates (metal, semiconductor, and insulator), and these include spin-casting, dropcasting, doctor-blading, self-assembly, etc. However, while these methods tend to form thick films, which can be glassy or crystalline solids, their precise composition cannot be tailored.223 These films have been studied to investigate ET between NQDs,224−228 but they are not desirable for fabrication of ETbased devices for light harvesting. Instead, bottom-up methods are desired to fabricate band-gap-graded structures. The main methods to fabricate films are described below, in which the composition and placement of each layer can be controlled. These include covalent grafting, electrostatic assembly, and assembly at the air−liquid interface. All of these techniques allow for control over the thickness, homogeneity, and composition of the layers and enable the easy fabrication of multilayer structures for ET applications. 5.1. Grafting of Nanocrystal Quantum Dots to Functionalized Surfaces

Figure 14. (a) Schematic representation of the grafting of NQDs to functionalized SAMs. Multilayer films are formed by repeating the NQD deposition and linker attachment steps. (b) SEM image of a single layer of CdSe/ZnS NQDs grafted onto an amine-terminated SAM on an oxide-free silicon substrate.

Functionalized SAMs on metallic and semiconducting substrates can covalently bind NQDs to the surface, as depicted in Figure 14a.229 Through grafting, the adhesion of the NQDs to the surface is improved compared to traditional deposition methods, including drop-casting and spin-coating, where the NQDs can be removed through rinsing and/or sonication in a compatible solvent. Covalent grafting of NQDs has been utilized on conductive surfaces for scanning tunneling microscopy and spectroscopy studies, as well as XPS, as the linking of NQDs to the substrate immobilizes the NQDs on substrate and enables charge dissipation.229−232 Immersion of the functionalized SAMs in dilute NQD solutions results in the adhesion of a single layer of NQDs to the surface (Figure 14b). The NQD density on the surface can be varied from single, well-isolated NQDs to dense coverage by controlling the immersion time and NQD solution concentration, but at all times it is less complete than a close-packed NQD monolayer. Many studies have utilized dithiols on Au,229 Ag,233 and GaAs234 surfaces, but other terminal functional groups can be used to bind NQDs, including amines217,230,235 and carboxylates.7 The choice of end group depends on the type of NQD and the solvent. For example, for water-soluble NQDs, carboxylate or aldehyde functionalities are utilized,7 while for nonpolar solvents, thiols or amines are common.229,235 The flexibility of the grafting technique allows the available substrates to be expanded to any on which a SAM can be assembled. The formation of SAMs on different substrates, including metals and semiconductors, has been extensively studied, and functionalized SAMs can be formed either directly with bifunctional molecules

or through multistep reactions.236,237 Additionally, by controlling the length of the alkyl chain in the SAM molecule, the distance between the NQD and substrate can be controlled.229 Another benefit of the grafting technique is that through sitespecific binding, the placement of the NQDs can be controlled. Spatial positioning of NQDs is essential for the fabrication of NQD-based devices. Through surface patterning, NQDs can be deposited in highly localized regions or patterned into twodimensional (2D) arrays.217,233,238 Seitz et al. showed that citrate-stabilized Au nanocrystals selectively bind to the active, oxide-free silicon region of FET sensors through the use of active (amine) and inactive (methyl) terminated SAMs on the silicon and oxide regions, respectively.217 Pacifico et al. utilized nanosphere lithographic patterning of a glass substrate to control the deposition of NQDs. Here, CdSe/ZnS core−shell NQDs selectively attach to the dithiol-passivated Ag islands, resulting in a long-range-ordered, 2D hexagonal pattern of grafted NQDs.233 Three-dimensional NQD structures can be assembled in a bottom-up manner through the use of bifunctional linkers to deposit multilayers of NQDs on both templated and nontemplated surfaces (Figure 14a). The linker molecules, such as dithiolated or diaminated alkyl chains, serve to bind two layers of NQDs together as one end of the linker binds to the already deposited NQDs while the other end remains free to bind the next layer of NQDs.220,234,239−242 Multilayers of NQDs are thus assembled though a cyclic process. In brief, SAM-terminated substrates are immersed in a NQD solution and rinsed, and then the NQD-coated surface is immersed in a linker solution and J

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rinsed again. The process of NQD attachment followed by linker passivation is repeated until the desire film thickness is achieved. Each immersion takes 1−2 h, so each layer takes 2−4 h to deposit. Using this technique, Rauf et al. formed thin films composed of up to 10 layers of the same size CdSe/ZnS NQDs,240 while other groups utilized this technique to assemble films with distinct sizes of NQDs in the different layers.220,234,241 For example, bilayer films in which small CdSe/ZnS NQDs were grafted to a monolayer of larger CdSe/ZnS NQDs via 1,6hexanediamine linkers were prepared to study the ET between the NQDs and into the underlying substrate.220 The linkers connecting the different layers can be modified to control the separation between layers by varying the alkyl chain length241 or to electronically couple the NQDs layers through use of conjugated dithiols.242 The deposition of NQD layers is studied via numerous techniques, including scanning electron microscopy (SEM), AFM, PL spectroscopy, and UV−vis absorbance spectroscopy. As multilayers are deposited, the increase in coverage is verified using PL, UV−vis, and AFM. The increase in PL counts is not linear and has been attributed to clustering and filter effects.239,240 Additionally, the binding of NQDs and its mechanism is studied via XPS.229,240

Figure 15. (a) Schematic representation of the deposition of NQDs via the LbL process. A positively charged substrate is (1) immersed in a solution of negatively charged NQDs, (2) rinsed, and then (3) immersed in a solution of positively charged monolayer and (4) rinsed. This results in the formation of a “bilayer.” Multilayer films are formed by repeating steps 1−4. (b) Schematic highlighting how the LbL process can be used to fabricate NQD multilayers. By using solutions of differently sized NQDs, a band-gap-graded structure can be assembled.

5.2. Layer-by-Layer Assembly

Uniform films of NQDs can be deposited on a variety of surfaces through layer-by-layer (LbL) assembly, where electrostatic interactions control the deposition and growth of the films. In brief, LbL films are assembled by exposing a charged substrate to alternating solutions of positively and negatively charged polymers or molecules and particles.243−245 While initially developed to assemble multilayers of polymeric materials,246 this technique is highly versatile and can be utilized for nanocrystals of different types (metal,247,248 semiconductor,222,249−251 magnetic,252,253 and metal oxide249,254), shapes (sphere, rods, plates, etc.),243 and sizes from nanocrystals249 to colloids.255 Additionally, the process can be carried out on flat, porous, or nonplanar substrates of various compositions, including semiconductors, insulators, or metals.249,254,256 NQD multilayers films assembled via the LbL method follow a standard procedure, as depicted in Figure 15a. In brief, NQD multilayers are assembled by repeating a two-step cycle where, first, the substrate is dipped into a polymer solution for ∼10 min and then rinsed with water and, next, the substrate is dipped into an aqueous NQD solution for ∼20 min and then rinsed with water. This process can be performed manually or automated with a robot. Each polymer/NQD cycle produces a “bilayer” structure, and the process can be repeated tens of times until the desired film thickness is achieved. The generality of the approach allows a variety of polymers and NQDs to be utilized. For polycationic polymers, molecules such as poly(allylamine hydrochloride) (PAH), poly(diallyldimethylammonium chloride) (PDDA), and poly(ethylenimine) (PEI) are often used, while for polyanionic polymers, species such as poly(acrylic acid) (PAA), poly(styrenesulfonate) (PSS), and poly(vinyl sulfate) (PVS) are utilized.243,245,246 The LbL process has been extensively reviewed,243,257 and here, we focus on the fabrication of NQD films for ET applications. For assembly of NQDs, the only requirement is that they have the opposite charge of the polymer. Aqueous syntheses of NQDs often utilize thioglycolic acid or 2mercaptoethylamine as ligands, and thus, the as-synthesized NQDs carry a negative or positive charge, respectively.258,259

However, most NQDs are synthesized according to organometallic procedures, resulting in neutral, nonpolar solutions of NQDs.260 Therefore, ligand exchanges must be performed to obtain charged NQDs.250,261,262 While studies typically utilize negatively charged QDs, positively charged NQDs can work as well; however, these are often less photostable.222 Through the combination of available polymers and NQDs, LbL assemblies can be rapidly fabricated with varying thickness, spacing, and composition for different applications. The deposition of NQDs can be monitored via absorbance and emission spectroscopies, where a linear increase is observed over tens of bilayers. This is indicative of uniform deposition in which the same amount of NQDs is deposited in each bilayer.249,251,263 The density of NQDs in a bilayer can be controlled through the immersion time. By varying the density, Lunz et al.264 examined the optical properties of NQD films as a function of a NQD’s interaction with its neighbors when they are well separated or close-packed. Additionally, the composition of the multilayer films can be controlled so that each bilayer consists of a differentsized NQD (Figure 15b). The formation of size gradient films allows excitons to be funneled from larger to smaller band gap NQDs and potent ially into t he underlying substrate.221,222,265−268 In these multilayer structures, the NQDs are separated by the polymer layer, which controls the distance between NQDs. As ET rates have a well-defined distance dependence, modifying the spacing between layers, typically from ∼0.5 to 1 nm, is of interest to control the rate and efficiency. Franzl et al.269 showed that it is possible to assembly bilayers of oppositely charged NQD directly. In this case, bilayers of CdTe NQDs capped with short alkyl chain carboxy- and amine-terminated thiols (negative and positive ligands, respectively) were assembled. The films exhibited a decreased interlayer distance and increased ET rate compared to the traditional polymer-assisted LbL assembly. However, this is not as successful as including a polymer interlayer, as issues with the balance of charges and NQD K

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Figure 16. (a) Schematic top−down and side views of a Langmuir−Blodgett trough. As the film is compressed, NQDs assemble on the surface of the water. Reprinted with permission from ref 283. Copyright 2008 American Chemical Society. (b) Schematic of NQD self-assembly at the air−liquid interface, where NQDs are deposited on a subphase, assemble as the solution evaporates, and are deposited on an underlying substrate. The rate of evaporation can be tuned by covering the well. Reprinted with permission from ref 285. Copyright 2010 Nature Publishing Group.

mobility arise.269 Alternatively, the distance between NQD layers can be increased by adding polymer bilayers. Cicek et al.270 showed that the emission wavelength of a film composed of two sizes of NQDs could be tuned by controlling the spacing between the NQD layers through the addition of one to five positive− negative polymer stacks. An additional degree of complexity can be added to LbL assemblies through the incorporation of molecules that interact with the NQDs. For example, Zhang et al.271 assembled hybrid organic/inorganic nanostructures films of CdSe/ZnS NQDs and J-aggregates using PDDA as the polycation between the NQD and J-aggregate. Here, the ability of the J-aggregates to harvest light increased the absorption of the NQDs due to energy transfer from J-aggregates to NQDs. The versatility of the LbL technique enables the design of hybrid structures where the interactions between layers can be precisely tuned.

then be transferred to a variety of substrates using vertical (Blodgett) or horizontal (Schaffere) lift-off.281,283,287,288 The technique is highly versatile and enables a great deal of control over the resulting NQD monolayer. The size, shape, and composition of the NQDs can be varied.282,283,287,289−292 In addition, by controlling the compression process, the packing density and interparticle distance can be tuned.293 Collier et al.294 showed that a reduction in the interparticle distance caused by higher compression induced the insulator-to-metal transition for a monolayer of silver nanocrystals. In evaporation-based self-assembly, the NQDs spontaneously assemble without any special equipment or application of forces as the solvent evaporates. Here, a colloidal solution of NQDs in toluene or hexane is deposited over a liquid subphase (water, acetonitrile, or diethylene glycol) either contained in a Teflon well or held together by surface tension on a hydrophobic surface, as shown in Figure 16b. As the solvent evaporates, the NQDs assemble into well-ordered, close-packed films. Then the monolayers can be transferred either by stamping the surface with a substrate or removing the subphase via extraction or evaporation, resulting in the monolayer settling on an underlying substrate.284,285,295 By controlling both the NQD concentration and the deposition volume, the thickness of the resulting film can be tuned from one to several monolayers.296 A benefit of this technique is the ability to assemble complex NQD monolayers. Long-range-ordered, single-component monolayers can be assembled from metal, semiconductor, and metal oxide nanocrystals by depositing a colloidal solution in toluene over water (Figure 17a).284,297 Changing the system to hexane and acetonitrile results in the formation of striped monolayers.295 On the other hand, by using a hexane and diethylene glycol system, single, binary, and even ternary component films can be fabricated (Figure 17b−d).285,296 Both Langmuir- and evaporation-driven self-assembly enable the formation of large (>cm2) areas of ordered NQD monolayers that can be easily transferred to solid substrates. Sequential transfers of NQD monolayers can be carried out where a linear increase in absorbance is observed.291 Compared to grafted or LbL structures, assembled monolayers have an increased packing density, which enables enhanced efficiency of Förster resonance ET interactions between neighboring NQDs.299 ET applications require layers of different NQDs, and as in the previous methods, bilayers of size-graded NQDs can be fabricated.299 However, a benefit of using assembly methods is the ability to fabricate more complex structures. Lambert et al.291 showed that “binary”

5.3. Self-Assembly of Nanocrystal Quantum Dot Monolayers at the Air−Liquid Interface

An alternative method for the deposition of NQD monolayers is self-assembly. Self-assembly of nanocrystals has been widely studied and enables the formation of single and multicomponent superlattices.260,272−276 Well-ordered arrays can be assembled, driven by weak forces, such as van der Waals attractions, hydrogen-bonding, hydrophobic interactions, and entropy.272,274,277 The resulting superlattices possess a high degree of structural and compositional diversity.278,279 However, the integration of NQD superlattices into devices remains as challenge, as large, uniform, two-dimensional arrays are needed. One successful method to fabricate large arrays is to carry out the self-assembly process at the air−liquid interface. This technique was initially studied by Langmuir and Blodgett for the assembly of surfactants on water280 and has been extended to the assembly of NQD monolayers.281,282 Two main techniques utilizing the air−liquid interface exist, where one is compression-driven while the other is evaporation-based. In both, a volatile, organic solution of NQDs is spread across the surface, where hydrophobic and/or immiscibility interactions enable the NQDs to float over the subphase surface.283−286 In the Langmuir technique, the solvent (often hexane, toluene, or chloroform) evaporates, leaving the NQDs spread across the subphase surface (usually water). A mobile barrier is used to slowly compress the NQDs until a dense monolayer is formed, as monitored via a Langmuir isotherm (surface pressure versus area), as schematically shown in Figure 16a. The monolayer can L

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propagate only within the energy-accepting semiconductor, that is, waveguide modes in spatially confined semiconductor layers and wires.311 These modes are later absorbed and generate electron−hole pairs, contributing to the electric current. The effective conversion of the incoming solar plane-wave photons, via re-emission from a relaxed localized exciton, into waveguide modes may be compared to the effect from scattering processes achievable with metallic and dielectric structures judiciously patterned to increase the incident light absorption in thin semiconductor layers.312,313 In order to accentuate the quantitative aspects of NRET and RET into layered planar Si structures and establish a theoretical perspective, we follow here a model analysis presented in earlier publications.7,301,302 The analysis considers how the decay rate of an electric-dipole emitter is modified in proximity to a planar interface, which can then be correlated with experimental observations of the emitter’s time-resolved PL (TRPL). There has been significant theoretical activity on this topic since Sommerfeld’s original treatment of the problem of an antenna’s emission near the earth’s surface.314 The problem is addressed in the framework of macroscopic electrodynamics,311,315 where electromagnetic decay rates and emission patterns of oscillating dipoles are calculated depending on the frequency (ω)dependent complex dielectric functions ε(ω) = ε′(ω) + iε″(ω) of the media surrounding the dipole. This approach has been applied to emitters in different environments, leading to good agreement with experimental data.311,315−317 In a vacuum, the lifetime τ0 (decay rate Γ0 = 1/τ0) of the spontaneous radiative decay of the emitter’s exciton311 is determined from the known

Figure 17. (a) SEM image of a monolayer of CdSe/ZnS NQDs on silicon assembled at the air−liquid interface using toluene and water. (b) SEM image of a ternary superlattice film self-assembled at the air−liquid interface using hexane and diethylene glycol from two sizes of Fe3O4 (16.5 and 7.0 nm) nanocrystals and 5.0 nm FePt nanocrystals. The colors indicate the three differently sized nanocrystals. Reprinted with permission from ref 296. Copyright 2011 American Chemical Society. TEM images of binary superlattice films self-assembled at the air−liquid interface using hexane and diethylene glycol from (c) CdSe/ZnS NQDs and Au/Fe3O4 nanocrystals and (d) PbSe/CdSe NQDs and Au/Fe3O4 nanocrystals. Reprinted with permission from ref 298. Copyright 2014 Wiley-VCH.

structures could be fabricated by controlling the size of NQDs in the different Langmuir layers. When the interparticle distance ratio of the larger to smaller NQDs is ∼√3, a structure is formed where each large particle is surrounded by six smaller particles. With the correct composition of NQDs, this close-packed structure could enhance ET between NQDs due to increased interactions with nearest neighbors. Cargnello et al.298 showed enhanced ET using self-assembled binary films of core−shell NQDs compared to simple mixtures.

Γ0 =

k3|p|2 3πε0ℏ

(5)

where k = ω/c = 2π/λ is the emission wavenumber and p is the (effective) dipole transition moment. The electromagnetic decay of the emitter changes in the vicinity of a planar interface, and the modified rate Γ can be expressed as

Γ/Γ0 = 1 + I(0, ∞)

6. ENERGY TRANSFER MECHANISMS FROM NANOCRYSTAL QUANTUM DOTS TO SILICON

(6)

For the randomly oriented transition dipole moment, averaging over its different orientations specifically yields eq 7:311

6.1. Theoretical Considerations

The main focus of this section is on the mechanisms of ET into Si from neighboring photoexcited quantum emitters, such as colloidal NQDs. We distinguish nonradiative ET (NRET) and radiative ET (RET)300 as qualitatively different contributions to the overall ET enabled by near-field electromagnetic interactions.7,235,301,302 Dexter303 was probably the first to explicitly suggest the idea of external NRET sensitization of inorganic semiconductors for PV applications. The NRET mechanism is akin to the familiar Förster ET (with the usual acronym FRET) enabled by dipole−dipole interactions between neighboring molecular species and depends on the spectral overlap of the energy-donor emission and energy-acceptor absorption spectra.304−306 The NRET process corresponds to direct excitation307 of an electron−hole pair in a semiconductor by the electric field of a decaying emitter’s exciton. While the NRET mechanism is frequently discussed in the literature for EThybrids,1,2,12,13,308−310 the potential significance of RET should not be overlooked, as its relative contribution varies depending on both the distance between the emitter and acceptor and the spectral range considered.7,302 In the RET process, the emitter’s exciton preferentially decays into photonic modes that can

I(a , b) = Re

∫a

b

u du 2 1−u

2

[(2u 2 − 1)r (p)(u) + r (s)(u)]

× exp(2ikz 1 − u 2 )

(7)

Here, z is the distance from the emitter to the substrate, the effect of which is contained in the reflection coefficients r(s) and r(p) for s- and p-polarized waves as determined by the substrate’s dielectric properties. The integration variable u in eq 7 defines the magnitude k∥ of the conserved in-plane components of the wave vector in terms of the vacuum wavenumber k: as k∥(u) = uk. The perpendicular components kz of the wave vector in a vacuum and silicon are, however, different and determined by kz12(u) = (1 − u 2)k 2 ,

kz 2 2(u) = (ε − u 2)k 2

(8)

respectively. As applied to different regions in space, we note that the real values of the perpendicular components in eq 8 signify the propagating waves while the imaginary values signify the evanescent ones. It is then instructive to divide eq 6 into parts for some ranges of variable u that would correspond to different decay channels. Figure 18 shows the numerical results from the M

DOI: 10.1021/acs.chemrev.5b00085 Chem. Rev. XXXX, XXX, XXX−XXX

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nonradiative process of Joule absorption of the oscillating dipolar electric fields in Si. The contributions from eqs 11 and 12 thus correspond to decay channels associated with ET into Si enabled by the near electromagnetic fields: eq 12 via direct generation of the electron−hole pairs and eq 11 via the subsequent absorption of the modes propagating in Si. Of course, a part (roughly, half301) of Γv in eq 10 propagates through the Si substrate and its absorption in Si would also contribute to overall radiative transfer. With prospective applications to ultrathin layers, this contribution is not added to our determination of ET efficiency. We thus define RET as due to exciton coupling only with evanescent fields in eq 11 and ET efficiency as ET efficiency =

direct computation of integrals in eq 7 using the dielectric function ε(ω) for Si that was reported by Aspnes and Studna.302,318 Figure 18a,b displays the ratio Γ/Γ0 in eq 6 computed for two distances, z = 4 and 6 nm, as a function of the emission wavelength. Displayed in the figure is the overall decay rate Γ = ΓSi, as well as its separate contributions: (9)

Different decay channels are defined and denoted here according to the following breakdown of eq 6: Γv /Γ0 = 1 + I(0,1)

(10)

ΓRET/Γ0 = I(1, ε′ )

(11)

ΓNRET/Γ0 = I( ε′ ,∞)

(12)

(13)

this quantity is shown in Figure 18d. Evidently, proximal excitons exhibit very high ET efficiencies. However, the efficacy of RET extends to longer distances; see the example of z = 50 nm in Figure 18d. Figure 18a,b shows that while Γv is lessened in proximity to the interface, ΓRET and ΓNRET become substantial, leading to an overall enhancement of the decay rate with respect to its vacuum value. The relative contributions of NRET and RET change depending on distance z and wavelength λ, as is seen in the examples in Figure 18c. Clearly, NRET is a dominant decay channel in the visible part of the spectrum at small spatial separations from Si. However, it becomes comparable to RET toward the near-infrared (NIR) and is less efficient than RET already at z = 6 nm, as shown in Figure 18c. While the detailed relationship between NRET and RET depends on numerical values of ε(ω), their different distance dependence leads to dominance of RET at sufficiently large distances from the substrate. For instance, at z ≥ 10 nm, NRET is practically negligible, while RET still makes a very appreciable contribution. It should be noted in Figure 18d that RET becomes increasly efficient toward the NIR due to smaller ratios z/λ. The same theoretical framework can be readily applied to emitters in the vicinity of thin Si layers, with the difference in the substrate structures being entirely expressed in the reflection coefficients in eq 7. The results of the illustrative calculation7 for a free-standing Si slab are shown in Figure 19. Panels a−c display the calculated dependence of the decay rate and various contributions as a function of the slab thickness (t) for three different distances (z) of the dipole emitter from the surface of the slab, and panel d emphasizes the dependence on spatial separation z. For these illustrations, the values of ε′ = 16 and ε″ = 0.25 were used, which approximately correspond318 to the Si dielectric function at 2.15 eV in the visible range. The simplified model clearly illustrates all the salient generic features without extra details of the structures studied experimentally. Just as in the results of Figure 18, one can see that the relative contributions of NRET and RET quickly change with distance z. NRET is dominant at a short distance z ≈ 4 nm (Figure 19a) but decreases to the rate of vacuum photon emission at z ≈ 9 nm (Figure 19b) and is not discernible in panel c. As NRET is appreciable only at distances of several nanometers, it is practically independent of the layer thickness in the displayed range. It is due to RET that ET into Si remains a dominant decay channel at large separations. Even at z ≈ 29 nm, as shown in Figure 19c, its rate is approximately 2 times larger than the rate of emission for photons that propagate in the whole space

Figure 18. Theoretical results illustrating modification of the decay rate as a function of vacuum wavelength for a randomly oriented electric dipole transition in the vicinity of the Si substrate described by the dielectric function ε(ω) reported by Aspnes and Studna.318 Upper panels display the total decay rate and its contributions from different channels in terms of the vacuum rate for distance (a) z = 4 nm and (b) z = 6 nm from the substrate. Panel c shows the ratio of contributions from NRET and RET for a series of indicated distances from the substrate. Panel d displays the corresponding total efficiencies of ET (eq 10) into the substrate for distances shown in panel c and, in addition, for larger separations as indicated. Reprinted with permission from ref 302. Copyright 2013 American Chemical Society.

ΓSi = Γv + ΓRET + ΓNRET

ΓRET + ΓNRET ΓSi

In this spectral range, the absorption in Si is weak, ε″ ≪ ε′, allowing for the clear separation into channels of eqs 10−12 (note that “propagating” modes are now weakly absorped in Si). In accordance with eq 8, eq 10 describes the decay into modes propagating in the whole space (vacuum and Si). Equation 11, however, describes the decay into modes propagating only inside Si, while on the vacuum side these modes are evanescent. Importantly, contributions from both eqs 10 and 11 correspond to radiative decay: those integrals do not vanish for ε″ = 0 and are practically independent for small ε″. However, the behavior of eq 12 is very different: that integral would be equal to zero for ε″ = 0 and is in fact proportional to small ε″. There are no propagating modes described by eq 12, instead it corresponds to the purely N

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distances z as long as 50 nm. Figure 19a−c also illustrates the oscillations of RET as a function of slab thickness t, and these oscillations are related to the strength of the dipole emission coupling to waveguiding modes in the slab. The oscillations occur with respect to the value that would take place for a dipole near the bulk Si substrate presented in Figure 18. Only for extremely thin layers does RET become noticeably smaller. Remarkably, these model results thus show that ET can be as efficient into ultrathin Si nanomebranes (perhaps as thin as ∼100−200 nm) as it is into bulk Si substrates. When combined with the efficacy of ET over a broad spectral range shown in Figure 18, these observations suggest that ET-sensitization of ultrathin Si layers may be utilized in PV devices. 6.2. Determination of NRET Efficiencies from Nanocrystal Quantum Dots to Semiconductor Substrates

The field of ET between molecular absorbers or semiconductor NQDs and various semiconductor surfaces for the purpose of photovoltaic light harvesting has seen something of resurgence in the past few years. Particularly, GaAs and silicon surfaces have been employed in order to improve efficiencies of the existing technologically important solar cells, where GaAs-based structures pertain to very high-efficiency (and possibly multijunction) cells while Si structures are thought to become more efficient for thin film geometries. Clear illustrations of the viability of the energy-transfer-based concepts first started to emerge in experimental studies of NQDs on GaAs substrates, in which the NRET efficiency was predicted to be substantial thanks to the large absorption coefficient of GaAs arising from its direct band gap transitions. The efficiencies of ET can be measured via temporally and spectrally resolved PL spectroscopy. This technique is a valuable tool that can be used to monitor the presence and dynamical behavior of correlated electrons and holes in semiconductor media and is extensively described in a number of books.319,320 In a typical PL system, the sample is excited by a laser [either in continuous (cw) or pulsed mode] promoting electron−hole pairs into higher energy states above the band gap of the semiconductor (Figure 20a). The excitations then undergo energy and momentum relaxation toward the band gap minimum, undergoing Coulomb scattering and interaction with phonons. Finally, the electrons recombine with holes, resulting in the emission of photons. The spectral position and time evolution of the emission (derived from TRPL measurements) provide valuable information about the nature of the electronic states in a semiconductor. Additionally, a number of

Figure 19. Model decay rates, in units of Γ0, of a randomly oriented pointlike electric dipole emitter at distance z from a free-standing Si layer. The dielectric function of Si used here for calculations is ε = 16 + 0.25i. Panels a−c display decay rates as a function of the layer thickness t and differ by the distance-to-the-layer, as measured in vacuum wavelengths λ: (a) z = 0.007λ, (b) z = 0.015λ, and (c) z = 0.05λ. The layer thickness is measured by the wavelengths in Si, λSi = λ/4. The solid black lines depict the total decay rate as expressed by eq 9. The contributions to this total rate are displayed as follows: green lines for ΓRET from the excitation of waveguiding modes in Si (eq 11), blue lines for Γv from the excitation of vacuum photons (eq 10), and red lines for ΓNRET from NRET into Si (eq 12). The benchmark results for the total decay rate in the vicinity of the bulk Si substrate are shown as black dashed-line levels. Panel d displays the dependence on distance z for several values of the layer thickness as indicated. NRET is the major contributor to Γ at distances smaller than ∼0.01λ, while RET dominates at larger separations up to ∼0.1λ. Reprinted with permission from ref 7. Copyright 2012 American Chemical Society.

(“vacuum photons”). Figure 19d displays the data for a broader range of distances. A closer inspection of the data shows that the rates of decay due to RET and vacuum photons are comparable at roughly distances z ∼ 0.1λ0. In the model example, this corresponds to approximately 50% efficiency of ET into Si from

Figure 20. (a) Schematic of the microscope-based PL system used to study ET from NQD to silicon surfaces. Manifestations of the ET process: (b) reduction of the PL intensity of the donor and simultaneous enhancement of the PL intensity of the acceptor and (c) shortening of the PL lifetime of the donor in the presence of the acceptor. Reprinted with permission from ref 301. Copyright 2013 Optical Society of America. O

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nonradiative transfer efficiency to the quantum wells from these NQDs. Another important achievement was the demonstration of a NRET-induced photocurrent in GaAs patterned quantum wells sensitized with CdSe/ZnS NQDs.310 By comparison of the PL decay times, the authors estimated the efficiency of NRET from NQDs to the proximal semiconductor surface to be 79%. Additionally, photocurrent measurements revealed a 6-fold enhancement for the patterned hybrid devices when compared to the reference p−i−n structure. From the data, the authors concluded that approximately 89% of the exciton energy was transferred into the quantum wells via NRET from the proximal NQDs. They also noted that such hybrid configurations are not limited to using NQDs as energy donors but should also be applicable to other strongly absorbing, solution-based materials. The studies discussed above were based on NQDs that were drop-cast onto semiconductor surfaces. In such situations, unprepared interfaces may have a substantial number of surface trapping sites that will facilitate exciton dissociation and subsequent charge trapping. This is especially important when TRPL dynamics are recorded as the method of choice to register NRET signaturesfaster dynamics of the emitting donor (NQD or molecule) species indicate an energy transfer channel. However, other nonradiative channels present such as interface charge trapping would similarly affect dynamics, thus hindering NRET effects. Further, drop-casting does not allow for uniform layer deposition, thus providing for a multitude of donor− acceptor distances, complicating interpretation of the PL lifetimes. Recently, several publications have achieved controllable positioning of monolayers of NQDs on Si surfaces, thus allowing for a quantitative determination of the energy transfer rates. Nguyen et al.235 discussed NRET from a monolayer of CdSe/ZnS NQDs controllably grafted on a SAM-passivated Si surface, as described in section 5.1. TRPL measurements revealed a progressive shortening of the PL lifetime of the donor NQDs as the separation from the acceptor Si surface decreased with ∼1/d3 dependence, confirming its dipole−dipole origin. The calculated NRET efficiency at 565 nm reached 65% with an interaction distance of about 4.4 nm. Extension of NRET to the near-infrared wavelength range was accomplished by Nimmo et al.,302 who similarly grafted a monolayer of CdSe/ZnS or CdSeTe/ZnS NQDs emitting between 545 and 800 nm on a Si surface. There, the efficiency of NRET was found to decrease from ∼70% to