Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group\u27s recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange

Self-organizing high-density single-walled carbon nanotube arrays from surfactant suspensions

Very thin films of oriented and densely packed single-walled carbon nanotubes (SWNTs) can be self-assembled on substrates from surfactant sodium dodecyl sulfate (SDS-) coated SWNT suspensions at ambient conditions. The evaporation of water causes a concentration of the SDS-coated nanotubes above critical micelle concentrations for SDS, and it is believed that self-organization of the SDS molecules serves as a driving force for the oriented and dense assembly of the nanotubes. The high degree of alignment in the SWNT thin films was characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and polarized Raman spectroscopy.link_to_subscribed_fulltex

Synthesis and characterization of (Ga1-xZnx)(N1-xOx) nanocrystals with a wide range of compositions

We describe the synthesis and characterization of wurtzite (Ga1-xZnx)(N1-xOx) nanocrystals with a wide range of compositions and a focus on properties relevant for solar fuel generation. (Ga1-xZnx)(N1-xOx), a solid solution of GaN and ZnO, is an intriguing material because it exhibits composition-dependent visible absorption even though the parent semiconductors absorb in the UV. When functionalized with co-catalysts, (Ga1-xZnx)(N1-xOx) is also capable of water splitting under visible irradiation. Here, we examine the synthesis of (Ga1-xZnx)(N1-xOx) nanocrystals to understand how they form by nitridation of ZnO and ZnGa2O4 nanocrystalline precursors. We find that the ZnO precursor is critical for the formation of crystalline (Ga1-xZnx)(N1-xOx) at 650 °C, consistent with a mechanism in which wurtzite (Ga1-xZnx)(N1-xOx) nucleates topotactically on wurtzite ZnO at an interface with ZnGa2O4. Using this information, we expand the range of compositions from previously reported 0.30 ≤ x ≤ 0.87 to include the low-x and high-x ends of the range. The resulting compositions, 0.06 ≤ x ≤ 0.98, constitute the widest range of (Ga1-xZnx)(N1-xOx) compositions obtained by one synthetic method. We then examine how the band gap depends on sample composition and find a minimum of 2.25 eV at x = 0.87, corresponding to a maximum possible solar-to-H2 power conversion efficiency of 12%. Finally, we examine the photoelectrochemical (PEC) oxidation behavior of thick films of (Ga1-xZnx)(N1-xOx) nanocrystals with x = 0.40, 0.52, and 0.87 under visible illumination. (Ga1-xZnx)(N1-xOx) nanocrystals with x = 0.40 exhibit solar PEC oxidation activity that, while too low for practical applications, is higher than that of bulk (Ga1-xZnx)(N1-xOx) of the same composition. The highest photocurrents are observed at x = 0.52, even though x = 0.87 absorbs more visible light, illustrating that the observed photocurrents are a result of an interplay of multiple parameters which remain to be elucidated. This set of characterizations provides information useful for future studies of composition-dependent PEC properties of nanoscale (Ga1-xZnx)(N1-xOx)

Synthesis and characterization of (Ga1-xZnx)(N1-xOx) nanocrystals with a wide range of compositions

We describe the synthesis and characterization of wurtzite (Ga1-xZnx)(N1-xOx) nanocrystals with a wide range of compositions and a focus on properties relevant for solar fuel generation. (Ga1-xZnx)(N1-xOx), a solid solution of GaN and ZnO, is an intriguing material because it exhibits composition-dependent visible absorption even though the parent semiconductors absorb in the UV. When functionalized with co-catalysts, (Ga1-xZnx)(N1-xOx) is also capable of water splitting under visible irradiation. Here, we examine the synthesis of (Ga1-xZnx)(N1-xOx) nanocrystals to understand how they form by nitridation of ZnO and ZnGa2O4 nanocrystalline precursors. We find that the ZnO precursor is critical for the formation of crystalline (Ga1-xZnx)(N1-xOx) at 650 °C, consistent with a mechanism in which wurtzite (Ga1-xZnx)(N1-xOx) nucleates topotactically on wurtzite ZnO at an interface with ZnGa2O4. Using this information, we expand the range of compositions from previously reported 0.30 ≤ x ≤ 0.87 to include the low-x and high-x ends of the range. The resulting compositions, 0.06 ≤ x ≤ 0.98, constitute the widest range of (Ga1-xZnx)(N1-xOx) compositions obtained by one synthetic method. We then examine how the band gap depends on sample composition and find a minimum of 2.25 eV at x = 0.87, corresponding to a maximum possible solar-to-H2 power conversion efficiency of 12%. Finally, we examine the photoelectrochemical (PEC) oxidation behavior of thick films of (Ga1-xZnx)(N1-xOx) nanocrystals with x = 0.40, 0.52, and 0.87 under visible illumination. (Ga1-xZnx)(N1-xOx) nanocrystals with x = 0.40 exhibit solar PEC oxidation activity that, while too low for practical applications, is higher than that of bulk (Ga1-xZnx)(N1-xOx) of the same composition. The highest photocurrents are observed at x = 0.52, even though x = 0.87 absorbs more visible light, illustrating that the observed photocurrents are a result of an interplay of multiple parameters which remain to be elucidated. This set of characterizations provides information useful for future studies of composition-dependent PEC properties of nanoscale (Ga1-xZnx)(N1-xOx)

Conductivity of single-walled carbon nanotubes probed by THz time-domain spectroscopy

THz time-domain spectroscopy is applied to probe the transient conductivity single-walled carbon nanotubes (SWNTs). The ultrafast transport properties and carrier dynamics are examined for different levels of laser excitation. © 2005 Optical Society of America