JPWO2008032618A1 - Semiconductor nanoparticles and manufacturing method thereof - Google Patents

Abstract

The semiconductor nanoparticle as a light emitting device material is converted from an indirect transition type to a direct transition type, and the quantum yield is improved. Modified semiconductor nanoparticles, wherein the slope of the tangent obtained by Tauc plot for the semiconductor nanoparticles is 2-5 of the slope of the tangent for bulk crystals of the same chemical composition as the core of the semiconductor nanoparticles It is characterized by being double.

Description

  The present invention relates to semiconductor nanoparticles and a method for producing the same. More specifically, the present invention relates to a semiconductor nanoparticle having an optical property converted from an indirect transition type to a direct transition type and an improved quantum yield as a light emitting device, and a method for manufacturing the same.

  In recent years, attention has been focused on nanostructure crystals exhibiting specific optical characteristics in II-VI group semiconductors such as ultrafine particles such as Si and Ge, and porous silicon. Here, the nanostructure crystal refers to a crystal grain having a nano-order particle size of about 1 to 100 nm, and is generally referred to as an abbreviation such as “nanoparticle” or “nanocrystal”.

  In the II-VI group semiconductor, when the nanostructure crystal as described above is compared with the case of having a bulk crystal, when the nanostructure crystal is included, good light absorption characteristics and light emission characteristics are exhibited. It will be. This is presumably because a II-VI group semiconductor having a nanostructure crystal has a larger band gap than a bulk crystal structure because a quantum size effect is exhibited. That is, in the II-VI group semiconductors having nanostructure crystals, it is considered that the band gap may be widened by the quantum size effect.

  By the way, semiconductors can be classified into two types according to the band gap type. That is, there are a direct transition type (direct type: gallium arsenide, etc.) in which light absorption and emission are simple, and an indirect transition type (indirect type: silicon, etc.) that requires a slightly complicated process for light absorption and emission.

  For example, crystalline silicon is an indirect transition type semiconductor having a band gap of 1.1 eV, and hydrogenated amorphous silicon has a band gap of about 1.5 to 1.7 eV although it depends on the hydrogen content. It is a direct transition type semiconductor. Solar cells made of amorphous silicon show an output voltage about 0.2-0.3 volts higher than crystalline silicon due to the deep band gap, whereas crystalline silicon is an indirect transition type, so its optical characteristics However, there are disadvantageous aspects in the production of light emitting elements.

  When nano-semiconductor particles are used as a light-emitting element, it is preferable that Si or Ge, which has no concern about toxicity and has a low raw material cost, be a semiconductor material component. However, semiconductors composed of components having few problems with respect to toxicity and the like are often indirect transition type, and have a disadvantage that the quantum yield is extremely low as a light emitting device material, which is a problem in practical use.

  Techniques for converting the indirect transition type to the direct transition type have been studied from various viewpoints, and some techniques have been disclosed (see, for example, Patent Documents 1 to 3). However, all of these known examples are methods of converting an indirect transition type to a direct transition type by laminating and bonding crystals having different chemical compositions, etc., and all are semiconductor elements formed on a substrate. Therefore, it is considered that application to a semiconductor nanoparticle system is difficult.

Therefore, research and development of conversion technologies applicable to semiconductor nanoparticle systems are desired.
JP-A-5-82837 Japanese Patent Laid-Open No. 7-79050 JP 2003-303983 A

  The present invention has been made in view of the above problems, and its solution is to convert the optical properties of semiconductor nanoparticles as a light emitting device material from an indirect transition type to a direct transition type, thereby improving the quantum yield. It is to provide semiconductor nanoparticles and a method for producing the same.

  The above-mentioned problem according to the present invention is solved by the following means.

  1. A semiconductor nanoparticle having an average particle diameter of 2 to 50 nm, the surface of which is modified, and the slope of the tangent obtained by Tauc plot for the semiconductor nanoparticle is the core of the semiconductor nanoparticle. 2 to 5 times the slope of the tangent for a bulk crystal having the same chemical composition.

  2. The band gap determined by the Tauc plot for the semiconductor nanoparticles is in the range of 0.2 to 1.5 eV with respect to the band gap determined by the Tauc plot for the bulk crystal having the same chemical composition as the core portion of the semiconductor nanoparticles. 2. The semiconductor nanoparticles according to 1 above, wherein the semiconductor nanoparticles are high.

  3. 3. The semiconductor nanoparticles as described in 1 or 2 above, wherein the semiconductor nanoparticles are core / shell type semiconductor nanoparticles.

  4). 4. The semiconductor nanoparticle according to any one of 1 to 3, wherein Si or Ge is contained as a constituent component of the semiconductor nanoparticle.

  5. A method for producing semiconductor nanoparticles according to any one of claims 1 to 4, comprising a step of subjecting the semiconductor nanoparticles to a surface treatment under a gas atmosphere.

  By the above means of the present invention, it is possible to provide a semiconductor nanoparticle having improved quantum yield and a method for producing the same by converting the optical property of the semiconductor nanoparticle as a light emitting device material from an indirect transition type to a direct transition type. .

Example of Tauc plot

Explanation of symbols

1 Tauc plot of Si bulk crystal 2 Tauc plot of sample heat-treated in Si atmosphere c + nitrogen atmosphere

  The semiconductor nanoparticles of the present invention are surface-modified semiconductor nanoparticles having an average particle diameter of 2 to 50 nm, and the slope of the tangent obtained by Tauc plot for the semiconductor nanoparticles is that of the semiconductor nanoparticles. It is characterized in that it is 2 to 5 times the gradient of the tangent for the bulk having the same chemical composition as the core.

  The core part of the semiconductor nanoparticle of this invention said here means the center part of the nanoparticle by which the surface was modified | denatured, and is synonymous with the semiconductor nanoparticle before modification | denaturation.

  Hereinafter, the present invention and its components will be described in detail.

(Semiconductor nanoparticles)
One semiconductor nanoparticle of the present invention is a semiconductor nanoparticle having a surface modified with an average particle diameter of 2 to 50 nm made of a semiconductor material, and a slope of a tangent obtained by Tauc plot for the semiconductor nanoparticle. The core portion of the semiconductor nanoparticle (semiconductor nanoparticle before modification) is 2 to 5 times the tangential slope of the bulk crystal having the same chemical composition.

(Core / shell type semiconductor nanoparticles)
One of the preferred embodiments of the semiconductor nanoparticles of the present invention is a so-called core having a core / shell structure in which the semiconductor nanoparticles are composed of a core portion made of a semiconductor material and a shell portion (shell layer) covering the core portion. / Shell-type semiconductor nanoparticles having an average particle diameter of 2 to 50 nm and a surface modified core / shell-type semiconductor nanoparticles, the slope of the tangent obtained by Tauc plot is the core part of the semiconductor nanoparticles 2 to 5 times the slope of the tangential line for a bulk crystal having the same chemical composition.

  The “Tauc plot” is a method for obtaining an optical band gap from an electron spectrum generally used for an amorphous semiconductor. In light absorption by optical transition between bands of an amorphous semiconductor, the relationship between absorbance and photon energy is expressed by the following equation.

α = k (E−E ₀ ) ² / E (k is a constant)
Where α is the absorbance, E is the photon energy, and E ₀ is the optical band gap. From this equation, the horizontal axis represents photon energy, the vertical axis represents the square root of the product of absorbance and photon energy, and a tangent line is drawn. The intersection of this tangent and the horizontal axis is the optical band gap (Tatsuo Shimizu “Amorphous Semiconductor”, Baifukan (1994), p201).

  Here, “bulk crystal” refers to a collection of particle crystals having a particle diameter of 1 μm or more.

  “Average particle size” refers to the cumulative 50% volume particle size measured by the laser scattering method.

  In the semiconductor nanoparticles having a modified surface according to the present invention, the difference between the tangent line obtained by the Tauc plot and the slope of the linear approximation line is within 5% for the bulk crystal having the same composition as the core part of the semiconductor nanoparticle. Is preferred. Here, the “linear approximation straight line” refers to a straight line obtained when linear approximation is performed on values in a range lower than the contact point between the Tauc plot and the obtained tangent line. Since the Tauc plot of the indirect transition type bulk crystal is almost a straight line, the difference in slope between the tangent line and the linear approximate straight line is within 5%.

  Further, in the semiconductor nanoparticles of the present invention, the bulk crystal having the same composition as the core part of the semiconductor nanoparticles, and in the core / shell type semiconductor nanoparticles, the same chemistry as the core part of the core / shell type semiconductor nanoparticles is used. About the bulk crystal of a composition, it is preferable that it is high in the range of 0.2-1.5 eV with respect to the band gap calculated | required by Tauc plot.

<Formation of core particles>
The semiconductor nanoparticles of the present invention or the core portion of the semiconductor nanoparticles having a core / shell structure can be formed using various known semiconductor materials.

  Examples of the semiconductor material used for the core include MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, GaAs, GaP, Examples include GaSb, InGaAs, InP, InN, InSb, InAs, AlAs, AlP, AlSb, AlS, PbS, PbSe, Ge, Si, and mixtures thereof. In the present invention, a particularly preferable semiconductor material is Si or Ge.

  The average particle size of the core part according to the present invention is preferably 1 to 40 nm in order to achieve the effect of the invention. More preferred is 2 to 30 nm.

  The “average particle diameter” of the core portion according to the present invention refers to a cumulative 50% volume particle diameter measured by a laser scattering method.

<Shell part>
The shell part which concerns on this invention is a layer which covers the said core part in the core / shell type semiconductor nanoparticle which concerns on this invention, and is a structure layer for forming a core / shell structure.

  Note that the shell portion according to the present invention may not completely cover the entire surface of the core particle as long as the core particle is not partially exposed to cause a harmful effect.

  As a semiconductor material used for the shell portion, various known semiconductor materials can be used. Specific examples include, for example, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaS, GaN, GaP, GaAs, GaSb, InAs, InN, InP, InSb, AlAs, AlN, AlP. , AlSb, or a mixture thereof.

In the present invention, particularly preferred semiconductor materials are SiO ₂ and ZnS.

<Method for producing fluorescent semiconductor fine particles>
Various conventionally known methods can be used for producing the semiconductor nanoparticles before modifying the surface of the semiconductor nanoparticles or core / shell type semiconductor nanoparticles of the present invention.

  As a production method of the liquid phase method, there are a precipitation method such as a coprecipitation method, a sol-gel method, a uniform precipitation method, and a reduction method. In addition, the reverse micelle method, the supercritical hydrothermal synthesis method, and the like are also excellent methods for producing nanoparticles (for example, JP 2002-322468, JP 2005-239775, JP 10-310770 A). No., JP 2000-104058 A, etc.).

  As a manufacturing method of the vapor phase method, (1) the opposing raw material semiconductor is evaporated by the first high-temperature plasma generated between the electrodes, and the second high-temperature plasma generated by electrodeless discharge in a reduced pressure atmosphere (2) A method of separating / removing nanoparticles from an anode made of a raw material semiconductor by electrochemical etching (for example, see Japanese Patent Application Laid-Open No. 2003-515459). ), A laser ablation method (for example, see JP-A No. 2004-356163) and the like are used. A method of synthesizing a powder containing particles by reacting a raw material gas in a gas phase in a low pressure state is also preferably used.

  As a method for producing semiconductor nanoparticles or core / shell type semiconductor nanoparticles according to the present invention, a production method in which core particles are produced by a liquid phase method and then the particles are coated with a shell material is preferable.

  In the present invention, as a method for modifying the surface of the semiconductor nanoparticles, the surface treatment of the semiconductor nanoparticles is performed in a gas atmosphere such as an oxygen atmosphere, an argon atmosphere, a nitrogen atmosphere, and a nitrogen + hydrogen atmosphere, It is necessary to optimize the conditions for satisfying the condition of the tangential slope obtained by the Tauc plot.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to this.
1. Preparation of core part Si particles Tetraoctyl ammonium bromide (TOAB) is added to 100 ml of toluene, and after stirring well, 92 μL of SiCl ₄ is dropped. After stirring for 1 hour, a reducing agent (2 ml of lithium aluminum hydride in THF (1M)) is added dropwise over 2 minutes. After leaving for 3 hours, 20 ml of methanol is added to deactivate excess reducing agent, thereby obtaining silicon nanoparticles in an organic solvent. By using a spray pyrolysis apparatus (Okawara Processing Machine Co., Ltd., RH-2), this is retained at 200 ° C. for 1 minute and dried to obtain silicon nanoparticle powder.

The particle size of the silicon nanoparticles can be adjusted by the ratio of SiCl ₄ and TOAB. SiCl ₄ : TOAB was changed from 1: 0.1 to 1: 100 to obtain monodisperse particles having the following four particle sizes.

SiCl ₄ : TOAB Average particle size Si nanoparticle a 1: 0.1 30 nm
Si nanoparticle b 1: 1 10nm
Si nanoparticle c 1:10 5 nm
Si nanoparticle d 1: 100 2 nm
2. Coating of shell part SiO ₂ layer Colloidal silica containing silicon dioxide (PL-3 manufactured by Fuso Chemical Industry Co., Ltd.) and potassium hydroxide mixed with pure water to adjust the liquid volume to 1500 ml, the above Si nanoparticles a ~ D is dispersed.

This dispersion was retained at 200 ° C. for 5 minutes using a spray pyrolysis apparatus to coat the SiO ₂ shell layer to obtain powders of Si / SiO ₂ core / shell nanoparticles A to D.
2. Surface treatment of nanoparticle powder The obtained Si nanoparticles a to d and Si / SiO ₂ core / shell nanoparticles A to D each having a shell layer coated thereon were mixed with oxygen atmosphere, argon atmosphere, nitrogen atmosphere, nitrogen + 1 The surface of the nanoparticles was modified at 900 ° C. while being retained for 10 minutes in a spray pyrolysis apparatus under different hydrogen atmosphere and conditions.

(Evaluation)
Tauc plot: For comparison, a visible ultraviolet absorption spectrum of a bulk Si substrate (thickness: 50 μm) is measured using a spectrophotometer. The visible ultraviolet absorption spectrum is measured for each of the obtained samples under exactly the same conditions. Take the photon energy on the horizontal axis, the square root of the product of absorbance and photon energy on the vertical axis, plot each data, and draw the tangent line. The intersection of this tangent and the horizontal axis is the optical band gap.

  FIG. 1 shows an example of Tauc plot. 1 is a Tauc plot of the Si bulk crystal, and 2 is a Tauc plot of a sample whose surface is modified by heat-treating the Si nanoparticles c in a nitrogen atmosphere, each showing its tangent line.

  Fluorescence quantum yield: About the obtained sample, the fluorescence spectrum which generate | occur | produces by irradiating the excitation light of wavelength 350nm was measured. The quantum yield was determined by comparing the molar extinction coefficient obtained from the absorption spectrum of the sample, the wavenumber integral value of the fluorescence spectrum, and the refractive index of the solvent with a standard substance (rhodamine B, anthracene, etc.) with a known quantum yield. .

When the quantum yield of the sample is φ _x and the quantum yield of the standard substance is φ _r , φ _x can be obtained by the following equation.
φ _x = F _x n _x ² / F _r n _r ² · ε _r cr _{r dr} / ε _x c _x d _x · φ _r (A)
Here, F _x is the wave number integration value of the sample, n _x is the refractive index of the standard solvent, ε _x c _x d _x is the absorbance of the sample, wavenumber integral value of F _r is standard, n _r is the standard The refractive index of the solvent, ε _r c _{r dr,} is the absorbance of the standard substance.

  The evaluation results are shown in Table 1.

  As is clear from the results shown in Table 1, the slope of the tangent obtained by Tauc plot for the surface-treated semiconductor nanoparticles or core / shell type semiconductor nanoparticles is the same as that of the core of the semiconductor nanoparticles. Particles satisfying the condition that the bulk crystal of the composition or the bulk crystal of the same chemical composition as the core part of the core / shell type semiconductor nanoparticles is 2 to 5 times the inclination of the tangent has a high fluorescence quantum yield. I understand that.

Claims (5)

Semiconductor nanoparticles with an average particle diameter of 2 to 50 nm and modified on the surface, the tangent slope obtained by Tauc plot for the semiconductor nanoparticles is the same as the core of the semiconductor nanoparticles before modification A semiconductor nanoparticle characterized by being 2 to 5 times the slope of the tangent to the bulk crystal. The band gap determined by the Tauc plot for the semiconductor nanoparticles is in the range of 0.2 to 1.5 eV with respect to the band gap determined by the Tauc plot for the bulk crystal having the same chemical composition as the core portion of the semiconductor nanoparticles. The semiconductor nanoparticles according to claim 1, wherein the semiconductor nanoparticles are high. The semiconductor nanoparticles according to claim 1 or 2, wherein the semiconductor nanoparticles are core / shell type semiconductor nanoparticles. The semiconductor nanoparticles according to any one of claims 1 to 3, wherein Si or Ge is contained as a constituent component of a core part of the semiconductor nanoparticles. A method for producing semiconductor nanoparticles according to any one of claims 1 to 4, comprising a step of subjecting the semiconductor nanoparticles to a surface treatment under a gas atmosphere. Particle production method.

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