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Electrical transport and grain growth in solution-cast, chloride-terminated cadmium selenide nanocrystal thin films.

Norman ZM, Anderson NC, Owen JS - ACS Nano (2014)

Bottom Line: Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200-300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C).Grain growth occurs at 200 °C in films with 8 atom % Cl(-), while films with 3 atom % Cl(-) resist growth until 300 °C.Fused nanocrystalline thin films (grain size = 4.5-5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm(2)/(Vs) and on/off ratios of 10(5) with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Columbia University , 3000 Broadway, MC 3121, New York, New York 10027, United States.

ABSTRACT
We report the evolution of electrical transport and grain size during the sintering of thin films spin-cast from soluble phosphine and amine-bound, chloride-terminated cadmium selenide nanocrystals. Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200-300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C). Grain growth occurs at 200 °C in films with 8 atom % Cl(-), while films with 3 atom % Cl(-) resist growth until 300 °C. Fused nanocrystalline thin films (grain size = 4.5-5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm(2)/(Vs) and on/off ratios of 10(5) with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

No MeSH data available.


Related in: MedlinePlus

Effect of annealing temperature on the in situ diffuse reflectance infrared and UV–visible absorption spectra of CdSe-CdCl2/PBu3 (A,C) and CdSe-CdCl2/NH2Bu (B,D). Near quantitative amine desorption occurs by 150 °C (B, inset) while >90% phosphine dissociation occurs by 200 °C (A, inset) (see Experimental Methods for details of quantitative analysis). Changes to the thin film asborption spectrum are nearly identical for both samples, where a significant red shift of the absorption onset and a broadening of the lowest energy transition (inset) occurs above 200 °C.
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fig1: Effect of annealing temperature on the in situ diffuse reflectance infrared and UV–visible absorption spectra of CdSe-CdCl2/PBu3 (A,C) and CdSe-CdCl2/NH2Bu (B,D). Near quantitative amine desorption occurs by 150 °C (B, inset) while >90% phosphine dissociation occurs by 200 °C (A, inset) (see Experimental Methods for details of quantitative analysis). Changes to the thin film asborption spectrum are nearly identical for both samples, where a significant red shift of the absorption onset and a broadening of the lowest energy transition (inset) occurs above 200 °C.

Mentions: With these two nanocrystal samples in hand, we investigated the thermal desorption of their L-type ligands using diffuse reflectance infrared spectroscopy (DRIFTS). Solutions of nanocrystals were slurried with anhydrous potassium bromide and loaded into an in situ DRIFTS cell that was evacuated to <10–4 Torr and heated while monitoring the vibrational spectrum (Figure 1). This approach allows us to use the absolute signal intensity as a quantitative measure of the relative organic ligand content during desorption. After 2 h under vacuum, the signals from the toluene solvent and free ligands are lost, leaving the signals from PBu3 and NH2Bu that are bound to the nanocrystal surface. Higher temperatures drive ligand desorption until 200 °C where 90% of the C–H stretching signal from CdSe-CdCl2/PBu3 disappears. By using the phosphine coverage measured in solution with NMR spectroscopy (32 phosphines/nanocrystal, 0.6 phosphines/nm2), we estimate that 2 ± 1 phosphines per nanocrystal remain (∼2 atom % carbon) after ligand desorption, an amount on the order of the starting [HPBu3]+[Cl]− content. In contrast, ≥99% of all signals in the C–H region from CdSe-CdCl2/NH2Bu are lost by 200 °C, corresponding to less than one amine ligand remaining per nanocrystal (<0.5 atom % carbon). EDX spectroscopy confirmed that nitrogen and phosphorus are eliminated after 2 h of annealing at 250 °C and 10–4 Torr.


Electrical transport and grain growth in solution-cast, chloride-terminated cadmium selenide nanocrystal thin films.

Norman ZM, Anderson NC, Owen JS - ACS Nano (2014)

Effect of annealing temperature on the in situ diffuse reflectance infrared and UV–visible absorption spectra of CdSe-CdCl2/PBu3 (A,C) and CdSe-CdCl2/NH2Bu (B,D). Near quantitative amine desorption occurs by 150 °C (B, inset) while >90% phosphine dissociation occurs by 200 °C (A, inset) (see Experimental Methods for details of quantitative analysis). Changes to the thin film asborption spectrum are nearly identical for both samples, where a significant red shift of the absorption onset and a broadening of the lowest energy transition (inset) occurs above 200 °C.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4216209&req=5

fig1: Effect of annealing temperature on the in situ diffuse reflectance infrared and UV–visible absorption spectra of CdSe-CdCl2/PBu3 (A,C) and CdSe-CdCl2/NH2Bu (B,D). Near quantitative amine desorption occurs by 150 °C (B, inset) while >90% phosphine dissociation occurs by 200 °C (A, inset) (see Experimental Methods for details of quantitative analysis). Changes to the thin film asborption spectrum are nearly identical for both samples, where a significant red shift of the absorption onset and a broadening of the lowest energy transition (inset) occurs above 200 °C.
Mentions: With these two nanocrystal samples in hand, we investigated the thermal desorption of their L-type ligands using diffuse reflectance infrared spectroscopy (DRIFTS). Solutions of nanocrystals were slurried with anhydrous potassium bromide and loaded into an in situ DRIFTS cell that was evacuated to <10–4 Torr and heated while monitoring the vibrational spectrum (Figure 1). This approach allows us to use the absolute signal intensity as a quantitative measure of the relative organic ligand content during desorption. After 2 h under vacuum, the signals from the toluene solvent and free ligands are lost, leaving the signals from PBu3 and NH2Bu that are bound to the nanocrystal surface. Higher temperatures drive ligand desorption until 200 °C where 90% of the C–H stretching signal from CdSe-CdCl2/PBu3 disappears. By using the phosphine coverage measured in solution with NMR spectroscopy (32 phosphines/nanocrystal, 0.6 phosphines/nm2), we estimate that 2 ± 1 phosphines per nanocrystal remain (∼2 atom % carbon) after ligand desorption, an amount on the order of the starting [HPBu3]+[Cl]− content. In contrast, ≥99% of all signals in the C–H region from CdSe-CdCl2/NH2Bu are lost by 200 °C, corresponding to less than one amine ligand remaining per nanocrystal (<0.5 atom % carbon). EDX spectroscopy confirmed that nitrogen and phosphorus are eliminated after 2 h of annealing at 250 °C and 10–4 Torr.

Bottom Line: Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200-300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C).Grain growth occurs at 200 °C in films with 8 atom % Cl(-), while films with 3 atom % Cl(-) resist growth until 300 °C.Fused nanocrystalline thin films (grain size = 4.5-5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm(2)/(Vs) and on/off ratios of 10(5) with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Columbia University , 3000 Broadway, MC 3121, New York, New York 10027, United States.

ABSTRACT
We report the evolution of electrical transport and grain size during the sintering of thin films spin-cast from soluble phosphine and amine-bound, chloride-terminated cadmium selenide nanocrystals. Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200-300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C). Grain growth occurs at 200 °C in films with 8 atom % Cl(-), while films with 3 atom % Cl(-) resist growth until 300 °C. Fused nanocrystalline thin films (grain size = 4.5-5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm(2)/(Vs) and on/off ratios of 10(5) with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

No MeSH data available.


Related in: MedlinePlus