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Surface chemistry, modification, and engineering of colloidal nanocrystals
Surface chemistry, modification, and engineering of colloidal nanocrystals
2
Bashiru Kayode Sodipo Department of Physics, Kaduna State University, Kaduna, Nigeria, Centre for Energy and Environmental Strategy Research, Kaduna State University, Kaduna, Nigeria
2.1
Introduction to the concept of surface engineering
All nanomaterials have a large surface-to-volume ratio, making their surfaces the dominant player in their physical and chemical properties. Surface ligands (molecules that bind to the surface) are essential components of nanomaterial synthesis, processing, and application [1, 2]. In the preparation and storage of nanostructures in colloidal form, the stability of the colloid is of utmost importance [3, 4]. In the absence of any surface coating, colloidal nanostructures exhibit surfaces that are unstable or reactive in aqueous environments. Hence, the nanomaterials agglomerate and form large clusters, resulting in increased particle size. They also exhibit the strong behavior of a bulk material counterpart [5]. Surface engineering is the process of modifying or coating the surface of a component to enhance its properties. The abundance of research tools that can engineer surfaces of materials and still maintain the integrity of the core interior material has unlocked a whole new frontier of research in the area of surface engineering [6, 7]. Surface engineering allows the fabrication of improved products with additional functionality to solid surfaces in order to enhance their application. Through surface engineering, the optical, electrical, mechanical, magnetic, and catalytic properties of a material can be modified to create material of choice, such as smart materials. Surface-engineered materials are now being developed for several applications, including photovoltaic devices, dye-sensitized solar cells, electronics, drug delivery, medical devices, optical photonic devices, negative refractive index devices, super lenses, artificial magnetism, cloaking devices, thermoelectric power generation, functional biological materials, and quantum cascade lasers. This chapter focuses on the surface chemistry, modification, and engineering of colloidal NCs. However, most concepts are applicable to other nanostructures.
Colloidal Metal Oxide Nanoparticles. https://doi.org/10.1016/B978-0-12-813357-6.00002-4 © 2020 Elsevier Inc. All rights reserved.
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2.2
Colloidal Metal Oxide Nanoparticles
History of development of surface modification and engineering
The history of surface modification of colloidal NCs is as old as the history of NCs. Colloidal NCs consist of an inorganic core with organic ligands bound to its surface [8]. Early photochemistry studies on tailored colloidal CdS and TiO2 NCs arose from the oil crisis in the late 1970s. The semiconductor NCs with enhanced surface chemistry were considered for efficient harvesting of solar energy by means of photoelectrochemistry [9]. However, the birth of modern nanoscience with nanocrystals (NCs) can be traced to the early 1980s through the present. Quantum confined colloidal semiconductor NCs were first synthesized with narrow size distributions in the 1990s [10]. From the beginning of the 2000s and until now, several NCs of metals, metal oxides, and semiconductors have been developed in the isotropic and anisotropic forms. Colloidal NCs with a semiconductor as the inorganic material, also known as quantum dots, exhibit size tunable band gaps and luminescence energies due to the quantum size effect [11]. The effort to fully harness their bright luminescence for several applications is influenced, by the surfactant ligands. The set of organic surfactants forms the capping layer and binding site, which screens the particle from agglomeration, controls nucleation and growth kinetics during synthesis, and determines the size, shape, and matrix compatibility of the NCs. The surface ligands provide the colloidal stabilization of the NCs through either steric or electrostatic stabilization. Rapid progress in the surface modification of the organic ligands of the colloidal NCs has led to their use in numerous applications. Modification and engineering of the ligands present on the surface of the NCs influence the physicochemical (optical and electronic) properties and determine the applications of the colloidal NCs [12].
2.3
Prevailing major principles
The surface plays an important role in the NC properties such as the solubility, reactivity, and stability. The surface ligands connect the NCs to other materials. This plays a vital role in the construction of superlattices in the fabrication of new devices and in the conjugation with target molecules for biomedical applications [13]. The physical and chemical properties of nanoscale materials are strongly affected by their surface chemistry. During the synthesis of colloidal NC precursors, surfactant and solvent are needed. Sometimes the surfactant acts also as a solvent. The adherence of surfactant molecules to the surfaces of growing NCs is one of the most important parameters influencing crystal growth. The organic surfactants used include alkyl phosphine oxides, alkyl phosphonic acids, alkyl phosphines, fatty acids, amines, and some nitrogen-containing aromatics. The surfactant molecules are coupling agents with metal coordinating and solvophilic groups [14]. The metal coordinating groups are electron donators that allow coordination to electron-poor metal atoms at the NC surface. It prevents further growth and aggregation through steric repulsion.
Surface chemistry, modification, and engineering of colloidal nanocrystals
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The solvophilic groups determine the solubility of the NCs in the solvent and also serves as the functional group that provides the binding surface site to the NCs. Modification of the surface ligands is often required for various NC applications in several areas including photovoltaic, optoelectronic, and biomedical. The ligands inhibit the performance of the inorganic core NCs. For applications such as optoelectronic, the ligands build an insulating barrier when the NCs are assembled into thin films, owing to the poor electric conductivity. The barrier layer of the ligands may block the charge injection, resulting in low carrier injection efficiency, poor charge transport balance, and inferior external quantum efficiency (EQE) [15]. The organic surfactants degrade upon transforming from colloidal solutions to solid thin films, which plagues device operation [16]. Similarly, for other NC applications such as biomedical, the common surface ligands are hydrophobic and therefore they are not directly soluble in aqueous solution. The hydrophobic surfactant molecules need to be modified with biofunctional molecules that are hydrophilic [17]. To achieve surface modification of the NCs for various applications, several coating strategies for the NCs can be found in the literature. The surface engineering strategy and coating materials used depend on the NC application. For biomedical applications, several coating materials, including amphiphillic phospholipids or polymers [18], silica [19], and thiol-containing organic acids [20], are reported. Also, for optoelectronic applications, surface-modifying shells such as ZnS [21] and ZnSe [22] can be found in the literature. The surface engineering principles can now be classified based on the applications and the coating materials used. The various surface engineering principles found in the literature can be classified as follows: 1. 2. 3. 4.
Chemical surface modification method via wet chemistry. Thermal decomposition method. Gas phase method. Plasma method.
2.4
Chemical surface modification via wet chemistry
Chemical surface modification via wet chemistry is one of the various surface engineering techniques found in the literature that is prepared in liquid phase. The various wet chemistry methods reported are summarily based on the ligand exchange method [23], layer by layer surface modification [24], wrapping an amphiphilic polymer around the NCs [25], bioconjugation [26], silanization with a silica shell [27], and binding of inorganic ligands (e.g., S2) to the nanocrystal surface through the colloidal atomic layer deposition (c-ALD) process [28]. The ligand exchange method is the most widely used wet chemistry method for modifying the NC surface. In a typical ligand exchange process, NCs are often dispersed in hexane or chloroform combined with the source of a new ligand at room temperature. The ligand exchange process can be carried out through mechanical stirring or heating of the mixture. For the former, the resulting mixture is allowed to react through mixing until NC precipitation is observed [29, 30]. Then, centrifugation is done to remove the
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Colloidal Metal Oxide Nanoparticles
BF4– d+
N
O
NOBF4/DMF
Ligands
~5 min
< 1 min
d + BF 4 d+
d+
BF4–
BF4–
Fig. 2.1 Schematic illustration of the ligand-exchange process with NOBF4. Reprinted with permission from A. Dong, et al., A generalized ligand-exchange strategy enabling sequential surface functionalization of colloidal nanocrystals. J. Am. Chem. Soc. 133(4) (2010) 998–1006. Copyright (2011) American Chemical Society.
supernatant. The precipitated NCs are redispersed in any hydrophilic media such as DMF, DMSO, or acetonitrile. The ligand-exchange reaction can also be carried out using a phase transfer process. In this procedure, the NCs in hexane dispersion were first combined with a polar solvent such as acetonitrile to form a two-phase mixture, into which an appropriate amount of the new ligand source will be added. The resulting mixture is then stirred until the NCs are transferred from the upper hexane layer to the bottom acetonitrile layer [29]. The surface-modified NCs can be purified through precipitation by adding toluene, and the precipitated NCs are redispersed in various hydrophilic media to form a stable colloidal dispersion, as shown in Fig. 2.1. For the latter, the ligand exchange process is carried out through heating of the mixture [31, 32]. The mixture will be heated under an inert atmosphere to reflux and the chloroform is allowed to evaporate. The remaining melt of the functionalized NCs is further heated to 130°C for a few hours. After the reaction time, the mixture is cooled to 60°C. And when any of the aforementioned polar solvents such as acetone is added, transparent-colored NC dispersion can be obtained.
2.5
Thermal decomposition (pyrolysis) method
The thermal decomposition method of modifying NCs is one of the prominent routes of coating NCs with higher band-gap inorganic materials. The inorganic materials have been shown to improve the photoluminescence quantum yields by passivating the surface nonradiative recombination sites [33]. NCs passivated with inorganic shell structures are more robust than organically passivated quantum dots. The inorganic shell can tolerate processing conditions for incorporation of the core NCs into solid-state structures without necessarily affecting the NC core. Examples of coreshell inorganic composite NC structures reported earlier include CdS on CdSe and CdSe on CdS [34], ZnS grown on CdS, and ZnS-coated CdSe [35]. The pyrolysis method has been used to prepare several inorganic coated NCs. Generally, the pyrolysis process involves the initial thermal decomposition of the organometallic
Surface chemistry, modification, and engineering of colloidal nanocrystals
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precursors of the NCs such as dimethylcadmium and trioctylphosphine selenide, in a coordinating solvent such as trioctylphosphine oxide (TOPO), as described previously [36]. The precursors will then be injected at temperatures ranging from 340°C to 360°C. Although the NC is grown at temperatures between 290°C and 300°C, to coat the NCs with the inorganic shell, the temperature of the pyrolysis reactor is allowed to cool down to a certain temperature suitable for the coating process. Subsequently, precursors of the coating materials such as diethylzinc (ZnEt2) and hexamethyldisilathiane ((TMS)2S) for Zn and S, respectively, are added to the pyrolysis reaction flask containing NCs dispersed in a suitable solvent such as tri-n-octylphosphine (TOP) or tri-n-octylphosphine oxide (TOPO). They are heated under an inert gas environment such as an atmosphere of N2 gas at a lower temperature. The temperature at which the precursors can be added ranged from 140°C to 220°C. When the desired temperature is reached, the Zn and S stock solutions are added drop-wise to the vigorously stirring reaction mixture over a period of 5–10 min. After the addition is completed, the mixture is allowed to cool to 90°C and left stirring for several hours. A 5 mL aliquot of butanol was added to the mixture to prevent the TOPO from solidifying upon cooling to room temperature [33, 37].
2.6
The gas phase method
The gas phase route is a method of surface modification and engineering of NCs in thin film for advanced materials applications such as photovoltaic and optoelectronics. It includes various methods of coating NCs through vaporization of the coating material precursors before deposition on the NCs. The methods include atomic layer deposition (ALD), chemical vapor deposition, and successive ionic layer adsorption and reaction (SILAR). ALD has sparked a good deal of interest due to its inherent benefits compared to other thin-film deposition techniques [38]. ALD is a widely used gas phase method of depositing high-quality dielectric, semiconducting, or metallic films on various substrates. Detailed descriptions of recent developments in ALD chemistry can be found in several reports [39–42]. Herein, the report on the use of ALD for modifying the surface of NCs is presented. ALD can be used to replace the bulky passivating ligands on the NCs by shorter molecules [43]. ALD is based on sequential exposure of a substrate surface to precursor vapors. This allows self-limiting reactions with surface functional groups, which gives room for coatings of high surface area materials such as NCs. The self-limiting aspect of ALD allows an excellent step for conformal deposition on high aspect ratio structures. Some surface areas react before others because of different precursor gas fluxes. However, the precursors can adsorb and subsequently desorb from the surface areas where the reaction has reached completion [40]. This allows precursors to react with other unreacted surface areas and produce a very conformal deposition. The self-terminating process in ALD enables angstrom-scale control over the coating thicknesses [44]. This is one of the advantages of ALD; it allows precise thickness
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Colloidal Metal Oxide Nanoparticles
control at the angstrom or monolayer level. ALD is able to meet the needs for atomic layer control and conformal deposition through the sequential self-limiting surface reactions process. In ALD, precursors are efficiently separated throughout the deposition process by pulsing a purge gas after each half-reaction to remove excess precursor from the chamber. As shown Fig. 2.2, the ALD of a binary compound AB requires a four step sequence: (i) deposition of A layer, (ii) purge to remove unreacted precursors for A, (iii) deposition of B layer that can react with A on the surface, and (iv) purge of B precursors before starting new cycle. Surface modification of NCs using the ALD method requires preparation and assembling of the NCs into thin films. NCs films are well suited for ALD because their open structure allows the diffusion of gaseous precursors. This allows an easy layerby-layer filling of interstices. Furthermore, pore sizes of the order of a few nanometers suggest that complete infiltration and filling should take place after only a few monolayers, indicating that this method could also be cost-effective [45]. The ALD method has been used to modify the surface of NCs through infill of NC thin films with metal oxides to produce nanocomposites in which the NCs are locked in place and protected against oxidative and photothermal damage [46]. Successive ionic layer adsorption and reaction (SILAR) is conceptually related to ALD, but the main difference between these techniques is, in SILAR no free precursors are allowed to be present after the completion of the half-reaction [28]. The amounts of precursors needed for each half-reaction are predetermined to match one monolayer coverage for all cores [47]. The precise knowledge of the total surface area of the cores present in the reaction mixture is required. This is a very challenging requirement for polydisperse or nonspherical cores [28]. The SILAR technique can also be used for the growth of complex colloidal semiconductor nanostructures, such as quantum shells and colloidal quantum wells [48].
A
Repeat
+ Products
B + Products
Fig. 2.2 Schematic representation of ALD using self-limiting surface chemistry and an AB binary reaction sequence. Reprinted with permission from S. George, A. Ott, J. Klaus, Surface chemistry for atomic layer growth. J. Phys. Chem. 100(31) (1996) 13121–13131. Copyright (1996) American Chemical Society.
Surface chemistry, modification, and engineering of colloidal nanocrystals
Fig. 2.3 Schematic of the radio frequency (RF) microplasma set-up. Reproduced from S. Mitra, et al., Temperature-dependent photoluminescence of surfaceengineered silicon nanocrystals. Sci. Rep. 6 (2016) 27727.
Gas inlet
RF Power electrode
Microplasma
21
Ground electrode Trigger electrode
NCs colloid
2.7
Plasma method
Few works have reported surface modifications of colloidal NCs using the plasma method [49]. Plasmas can be generated by supplying energy to a neutral gas, causing the formation of charge carriers [50]. As shown in Fig. 2.3, plasma generated between the RF electrode and the ground electrode within a quartz capillary is used to modify the surface of NCs. A trigger electrode placed near the ground electrode was used to facilitate plasma ignition at reduced power [51]. Pure helium gas is flown inside the quartz capillary at the rate of 250 sccm. The applied power supply was kept at 60 W (450 MHz) and the reflected power was around 5 W. During the surface modification process, the distance between the end of the quartz capillary and the surface of the colloid is adjusted. Due to the unique conditions generated by microplasma, the process enhances photoluminescence intensity of the passivated NCs [52].
2.8
Conclusion
This chapter presents surface modification and surface engineering of the NC surface. The physical and chemical properties of NC materials are strongly affected by their surface chemistry. So, to exploit and apply NCs for various applications, surface modification of the nanomaterials is required. The history of surface engineering is summarized. The prevailing surface engineering methods reported in the literature are grouped into four classes based on their principles and chemistries. The techniques are wet chemistry, thermal decomposition, gas phase, and plasma methods. The methods are basically influenced by the choice of surface materials and applications of the NCs.
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Acknowledgments The author will like to acknowledge the contribution of S.J. Babalola, a PhD candidate of Universiti Sains Malaysia and staff of Modibbo Adama University of Technology, Yola, for his contribution toward preparing this chapter. He assisted in accessing the relevant and reference materials.
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37. M.A. Hines, P. Guyot-Sionnest, Synthesis and characterization of strongly luminescing ZnS-capped CdSe nanocrystals, J. Phys. Chem. 100 (2) (1996) 468–471. 38. H. Kim, W.-J. Maeng, Applications of atomic layer deposition to nanofabrication and emerging nanodevices, Thin Solid Films 517 (8) (2009) 2563–2580. 39. R.W. Johnson, A. Hultqvist, S.F. Bent, A brief review of atomic layer deposition: from fundamentals to applications, Mater. Today 17 (5) (2014) 236–246. 40. S.M. George, Atomic layer deposition: an overview, Chem. Rev. 110 (1) (2009) 111–131. 41. M. Knez, K. Nielsch, L. Niinist€o, Synthesis and surface engineering of complex nanostructures by atomic layer deposition, Adv. Mater. 19 (21) (2007) 3425–3438. 42. M. Leskel€a, M. Ritala, Atomic layer deposition (ALD): from precursors to thin film structures, Thin Solid Films 409 (1) (2002) 138–146. 43. S. ten Cate, et al., Activating carrier multiplication in PbSe quantum dot solids by infilling with atomic layer deposition, J. Phys. Chem. Lett. 4 (11) (2013) 1766–1770. 44. C. Marichy, M. Bechelany, N. Pinna, Atomic layer deposition of nanostructured materials for energy and environmental applications, Adv. Mater. 24 (8) (2012) 1017–1032. 45. A. Pourret, P. Guyot-Sionnest, J.W. Elam, Atomic layer deposition of ZnO in quantum dot thin films, Adv. Mater. 21 (2) (2009) 232–235. 46. Y. Liu, et al., Robust, functional nanocrystal solids by infilling with atomic layer deposition, Nano Lett. 11 (12) (2011) 5349–5355. 47. A.B. Greytak, et al., Alternating layer addition approach to CdSe/CdS core/shell quantum dots with near-unity quantum yield and high on-time fractions, Chem. Sci. 3 (6) (2012) 2028–2034. 48. J.J. Li, et al., Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction, J. Am. Chem. Soc. 125 (41) (2003) 12567–12575. 49. V. Sˇvrcek, et al., Stable ultrathin surfactant-free surface-engineered silicon nanocrystal solar cells deposited at room temperature, Energy Sci. Eng. 5 (4) (2017) 184–193. 50. H. Conrads, M. Schmidt, Plasma generation and plasma sources, Plasma Sources Sci. Technol. 9 (4) (2000) 441. 51. S. Mitra, et al., Temperature-dependent photoluminescence of surface-engineered silicon nanocrystals, Sci. Rep. 6 (2016) 27727. 52. V. Sˇvrcek, D. Mariotti, M. Kondo, Microplasma-induced surface engineering of silicon nanocrystals in colloidal dispersion, Appl. Phys. Lett. 97 (16) (2010) 161502.
Further reading 53. S. George, A. Ott, J. Klaus, Surface chemistry for atomic layer growth, J. Phys. Chem. 100 (31) (1996) 13121–13131.
2
Bashiru Kayode Sodipo Department of Physics, Kaduna State University, Kaduna, Nigeria, Centre for Energy and Environmental Strategy Research, Kaduna State University, Kaduna, Nigeria
2.1
Introduction to the concept of surface engineering
All nanomaterials have a large surface-to-volume ratio, making their surfaces the dominant player in their physical and chemical properties. Surface ligands (molecules that bind to the surface) are essential components of nanomaterial synthesis, processing, and application [1, 2]. In the preparation and storage of nanostructures in colloidal form, the stability of the colloid is of utmost importance [3, 4]. In the absence of any surface coating, colloidal nanostructures exhibit surfaces that are unstable or reactive in aqueous environments. Hence, the nanomaterials agglomerate and form large clusters, resulting in increased particle size. They also exhibit the strong behavior of a bulk material counterpart [5]. Surface engineering is the process of modifying or coating the surface of a component to enhance its properties. The abundance of research tools that can engineer surfaces of materials and still maintain the integrity of the core interior material has unlocked a whole new frontier of research in the area of surface engineering [6, 7]. Surface engineering allows the fabrication of improved products with additional functionality to solid surfaces in order to enhance their application. Through surface engineering, the optical, electrical, mechanical, magnetic, and catalytic properties of a material can be modified to create material of choice, such as smart materials. Surface-engineered materials are now being developed for several applications, including photovoltaic devices, dye-sensitized solar cells, electronics, drug delivery, medical devices, optical photonic devices, negative refractive index devices, super lenses, artificial magnetism, cloaking devices, thermoelectric power generation, functional biological materials, and quantum cascade lasers. This chapter focuses on the surface chemistry, modification, and engineering of colloidal NCs. However, most concepts are applicable to other nanostructures.
Colloidal Metal Oxide Nanoparticles. https://doi.org/10.1016/B978-0-12-813357-6.00002-4 © 2020 Elsevier Inc. All rights reserved.
16
2.2
Colloidal Metal Oxide Nanoparticles
History of development of surface modification and engineering
The history of surface modification of colloidal NCs is as old as the history of NCs. Colloidal NCs consist of an inorganic core with organic ligands bound to its surface [8]. Early photochemistry studies on tailored colloidal CdS and TiO2 NCs arose from the oil crisis in the late 1970s. The semiconductor NCs with enhanced surface chemistry were considered for efficient harvesting of solar energy by means of photoelectrochemistry [9]. However, the birth of modern nanoscience with nanocrystals (NCs) can be traced to the early 1980s through the present. Quantum confined colloidal semiconductor NCs were first synthesized with narrow size distributions in the 1990s [10]. From the beginning of the 2000s and until now, several NCs of metals, metal oxides, and semiconductors have been developed in the isotropic and anisotropic forms. Colloidal NCs with a semiconductor as the inorganic material, also known as quantum dots, exhibit size tunable band gaps and luminescence energies due to the quantum size effect [11]. The effort to fully harness their bright luminescence for several applications is influenced, by the surfactant ligands. The set of organic surfactants forms the capping layer and binding site, which screens the particle from agglomeration, controls nucleation and growth kinetics during synthesis, and determines the size, shape, and matrix compatibility of the NCs. The surface ligands provide the colloidal stabilization of the NCs through either steric or electrostatic stabilization. Rapid progress in the surface modification of the organic ligands of the colloidal NCs has led to their use in numerous applications. Modification and engineering of the ligands present on the surface of the NCs influence the physicochemical (optical and electronic) properties and determine the applications of the colloidal NCs [12].
2.3
Prevailing major principles
The surface plays an important role in the NC properties such as the solubility, reactivity, and stability. The surface ligands connect the NCs to other materials. This plays a vital role in the construction of superlattices in the fabrication of new devices and in the conjugation with target molecules for biomedical applications [13]. The physical and chemical properties of nanoscale materials are strongly affected by their surface chemistry. During the synthesis of colloidal NC precursors, surfactant and solvent are needed. Sometimes the surfactant acts also as a solvent. The adherence of surfactant molecules to the surfaces of growing NCs is one of the most important parameters influencing crystal growth. The organic surfactants used include alkyl phosphine oxides, alkyl phosphonic acids, alkyl phosphines, fatty acids, amines, and some nitrogen-containing aromatics. The surfactant molecules are coupling agents with metal coordinating and solvophilic groups [14]. The metal coordinating groups are electron donators that allow coordination to electron-poor metal atoms at the NC surface. It prevents further growth and aggregation through steric repulsion.
Surface chemistry, modification, and engineering of colloidal nanocrystals
17
The solvophilic groups determine the solubility of the NCs in the solvent and also serves as the functional group that provides the binding surface site to the NCs. Modification of the surface ligands is often required for various NC applications in several areas including photovoltaic, optoelectronic, and biomedical. The ligands inhibit the performance of the inorganic core NCs. For applications such as optoelectronic, the ligands build an insulating barrier when the NCs are assembled into thin films, owing to the poor electric conductivity. The barrier layer of the ligands may block the charge injection, resulting in low carrier injection efficiency, poor charge transport balance, and inferior external quantum efficiency (EQE) [15]. The organic surfactants degrade upon transforming from colloidal solutions to solid thin films, which plagues device operation [16]. Similarly, for other NC applications such as biomedical, the common surface ligands are hydrophobic and therefore they are not directly soluble in aqueous solution. The hydrophobic surfactant molecules need to be modified with biofunctional molecules that are hydrophilic [17]. To achieve surface modification of the NCs for various applications, several coating strategies for the NCs can be found in the literature. The surface engineering strategy and coating materials used depend on the NC application. For biomedical applications, several coating materials, including amphiphillic phospholipids or polymers [18], silica [19], and thiol-containing organic acids [20], are reported. Also, for optoelectronic applications, surface-modifying shells such as ZnS [21] and ZnSe [22] can be found in the literature. The surface engineering principles can now be classified based on the applications and the coating materials used. The various surface engineering principles found in the literature can be classified as follows: 1. 2. 3. 4.
Chemical surface modification method via wet chemistry. Thermal decomposition method. Gas phase method. Plasma method.
2.4
Chemical surface modification via wet chemistry
Chemical surface modification via wet chemistry is one of the various surface engineering techniques found in the literature that is prepared in liquid phase. The various wet chemistry methods reported are summarily based on the ligand exchange method [23], layer by layer surface modification [24], wrapping an amphiphilic polymer around the NCs [25], bioconjugation [26], silanization with a silica shell [27], and binding of inorganic ligands (e.g., S2) to the nanocrystal surface through the colloidal atomic layer deposition (c-ALD) process [28]. The ligand exchange method is the most widely used wet chemistry method for modifying the NC surface. In a typical ligand exchange process, NCs are often dispersed in hexane or chloroform combined with the source of a new ligand at room temperature. The ligand exchange process can be carried out through mechanical stirring or heating of the mixture. For the former, the resulting mixture is allowed to react through mixing until NC precipitation is observed [29, 30]. Then, centrifugation is done to remove the
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BF4– d+
N
O
NOBF4/DMF
Ligands
~5 min
< 1 min
d + BF 4 d+
d+
BF4–
BF4–
Fig. 2.1 Schematic illustration of the ligand-exchange process with NOBF4. Reprinted with permission from A. Dong, et al., A generalized ligand-exchange strategy enabling sequential surface functionalization of colloidal nanocrystals. J. Am. Chem. Soc. 133(4) (2010) 998–1006. Copyright (2011) American Chemical Society.
supernatant. The precipitated NCs are redispersed in any hydrophilic media such as DMF, DMSO, or acetonitrile. The ligand-exchange reaction can also be carried out using a phase transfer process. In this procedure, the NCs in hexane dispersion were first combined with a polar solvent such as acetonitrile to form a two-phase mixture, into which an appropriate amount of the new ligand source will be added. The resulting mixture is then stirred until the NCs are transferred from the upper hexane layer to the bottom acetonitrile layer [29]. The surface-modified NCs can be purified through precipitation by adding toluene, and the precipitated NCs are redispersed in various hydrophilic media to form a stable colloidal dispersion, as shown in Fig. 2.1. For the latter, the ligand exchange process is carried out through heating of the mixture [31, 32]. The mixture will be heated under an inert atmosphere to reflux and the chloroform is allowed to evaporate. The remaining melt of the functionalized NCs is further heated to 130°C for a few hours. After the reaction time, the mixture is cooled to 60°C. And when any of the aforementioned polar solvents such as acetone is added, transparent-colored NC dispersion can be obtained.
2.5
Thermal decomposition (pyrolysis) method
The thermal decomposition method of modifying NCs is one of the prominent routes of coating NCs with higher band-gap inorganic materials. The inorganic materials have been shown to improve the photoluminescence quantum yields by passivating the surface nonradiative recombination sites [33]. NCs passivated with inorganic shell structures are more robust than organically passivated quantum dots. The inorganic shell can tolerate processing conditions for incorporation of the core NCs into solid-state structures without necessarily affecting the NC core. Examples of coreshell inorganic composite NC structures reported earlier include CdS on CdSe and CdSe on CdS [34], ZnS grown on CdS, and ZnS-coated CdSe [35]. The pyrolysis method has been used to prepare several inorganic coated NCs. Generally, the pyrolysis process involves the initial thermal decomposition of the organometallic
Surface chemistry, modification, and engineering of colloidal nanocrystals
19
precursors of the NCs such as dimethylcadmium and trioctylphosphine selenide, in a coordinating solvent such as trioctylphosphine oxide (TOPO), as described previously [36]. The precursors will then be injected at temperatures ranging from 340°C to 360°C. Although the NC is grown at temperatures between 290°C and 300°C, to coat the NCs with the inorganic shell, the temperature of the pyrolysis reactor is allowed to cool down to a certain temperature suitable for the coating process. Subsequently, precursors of the coating materials such as diethylzinc (ZnEt2) and hexamethyldisilathiane ((TMS)2S) for Zn and S, respectively, are added to the pyrolysis reaction flask containing NCs dispersed in a suitable solvent such as tri-n-octylphosphine (TOP) or tri-n-octylphosphine oxide (TOPO). They are heated under an inert gas environment such as an atmosphere of N2 gas at a lower temperature. The temperature at which the precursors can be added ranged from 140°C to 220°C. When the desired temperature is reached, the Zn and S stock solutions are added drop-wise to the vigorously stirring reaction mixture over a period of 5–10 min. After the addition is completed, the mixture is allowed to cool to 90°C and left stirring for several hours. A 5 mL aliquot of butanol was added to the mixture to prevent the TOPO from solidifying upon cooling to room temperature [33, 37].
2.6
The gas phase method
The gas phase route is a method of surface modification and engineering of NCs in thin film for advanced materials applications such as photovoltaic and optoelectronics. It includes various methods of coating NCs through vaporization of the coating material precursors before deposition on the NCs. The methods include atomic layer deposition (ALD), chemical vapor deposition, and successive ionic layer adsorption and reaction (SILAR). ALD has sparked a good deal of interest due to its inherent benefits compared to other thin-film deposition techniques [38]. ALD is a widely used gas phase method of depositing high-quality dielectric, semiconducting, or metallic films on various substrates. Detailed descriptions of recent developments in ALD chemistry can be found in several reports [39–42]. Herein, the report on the use of ALD for modifying the surface of NCs is presented. ALD can be used to replace the bulky passivating ligands on the NCs by shorter molecules [43]. ALD is based on sequential exposure of a substrate surface to precursor vapors. This allows self-limiting reactions with surface functional groups, which gives room for coatings of high surface area materials such as NCs. The self-limiting aspect of ALD allows an excellent step for conformal deposition on high aspect ratio structures. Some surface areas react before others because of different precursor gas fluxes. However, the precursors can adsorb and subsequently desorb from the surface areas where the reaction has reached completion [40]. This allows precursors to react with other unreacted surface areas and produce a very conformal deposition. The self-terminating process in ALD enables angstrom-scale control over the coating thicknesses [44]. This is one of the advantages of ALD; it allows precise thickness
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control at the angstrom or monolayer level. ALD is able to meet the needs for atomic layer control and conformal deposition through the sequential self-limiting surface reactions process. In ALD, precursors are efficiently separated throughout the deposition process by pulsing a purge gas after each half-reaction to remove excess precursor from the chamber. As shown Fig. 2.2, the ALD of a binary compound AB requires a four step sequence: (i) deposition of A layer, (ii) purge to remove unreacted precursors for A, (iii) deposition of B layer that can react with A on the surface, and (iv) purge of B precursors before starting new cycle. Surface modification of NCs using the ALD method requires preparation and assembling of the NCs into thin films. NCs films are well suited for ALD because their open structure allows the diffusion of gaseous precursors. This allows an easy layerby-layer filling of interstices. Furthermore, pore sizes of the order of a few nanometers suggest that complete infiltration and filling should take place after only a few monolayers, indicating that this method could also be cost-effective [45]. The ALD method has been used to modify the surface of NCs through infill of NC thin films with metal oxides to produce nanocomposites in which the NCs are locked in place and protected against oxidative and photothermal damage [46]. Successive ionic layer adsorption and reaction (SILAR) is conceptually related to ALD, but the main difference between these techniques is, in SILAR no free precursors are allowed to be present after the completion of the half-reaction [28]. The amounts of precursors needed for each half-reaction are predetermined to match one monolayer coverage for all cores [47]. The precise knowledge of the total surface area of the cores present in the reaction mixture is required. This is a very challenging requirement for polydisperse or nonspherical cores [28]. The SILAR technique can also be used for the growth of complex colloidal semiconductor nanostructures, such as quantum shells and colloidal quantum wells [48].
A
Repeat
+ Products
B + Products
Fig. 2.2 Schematic representation of ALD using self-limiting surface chemistry and an AB binary reaction sequence. Reprinted with permission from S. George, A. Ott, J. Klaus, Surface chemistry for atomic layer growth. J. Phys. Chem. 100(31) (1996) 13121–13131. Copyright (1996) American Chemical Society.
Surface chemistry, modification, and engineering of colloidal nanocrystals
Fig. 2.3 Schematic of the radio frequency (RF) microplasma set-up. Reproduced from S. Mitra, et al., Temperature-dependent photoluminescence of surfaceengineered silicon nanocrystals. Sci. Rep. 6 (2016) 27727.
Gas inlet
RF Power electrode
Microplasma
21
Ground electrode Trigger electrode
NCs colloid
2.7
Plasma method
Few works have reported surface modifications of colloidal NCs using the plasma method [49]. Plasmas can be generated by supplying energy to a neutral gas, causing the formation of charge carriers [50]. As shown in Fig. 2.3, plasma generated between the RF electrode and the ground electrode within a quartz capillary is used to modify the surface of NCs. A trigger electrode placed near the ground electrode was used to facilitate plasma ignition at reduced power [51]. Pure helium gas is flown inside the quartz capillary at the rate of 250 sccm. The applied power supply was kept at 60 W (450 MHz) and the reflected power was around 5 W. During the surface modification process, the distance between the end of the quartz capillary and the surface of the colloid is adjusted. Due to the unique conditions generated by microplasma, the process enhances photoluminescence intensity of the passivated NCs [52].
2.8
Conclusion
This chapter presents surface modification and surface engineering of the NC surface. The physical and chemical properties of NC materials are strongly affected by their surface chemistry. So, to exploit and apply NCs for various applications, surface modification of the nanomaterials is required. The history of surface engineering is summarized. The prevailing surface engineering methods reported in the literature are grouped into four classes based on their principles and chemistries. The techniques are wet chemistry, thermal decomposition, gas phase, and plasma methods. The methods are basically influenced by the choice of surface materials and applications of the NCs.
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Acknowledgments The author will like to acknowledge the contribution of S.J. Babalola, a PhD candidate of Universiti Sains Malaysia and staff of Modibbo Adama University of Technology, Yola, for his contribution toward preparing this chapter. He assisted in accessing the relevant and reference materials.
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Further reading 53. S. George, A. Ott, J. Klaus, Surface chemistry for atomic layer growth, J. Phys. Chem. 100 (31) (1996) 13121–13131.