IEC 62607 3 1 Edition 1 0 2014 05 INTERNATIONAL STANDARD NORME INTERNATIONALE Nanomanufacturing – Key control characteristics Part 3 1 Luminescent nanomaterials – Quantum efficiency Nanofabrication –[.]
General
To ensure accurate measurements of the quantum efficiency of luminescent nanomaterials, it is essential to adhere to good laboratory practices Specifically, the measurement area must be kept clean and devoid of dirt and debris, along with any potential sources of contamination.
Ambient onditions
Test equipment must be placed in an environment with a stable temperature of 25 ± 2 °C, consistent relative humidity, and steady air flow It is essential to avoid locations near heating, ventilation, or air conditioning vents, as well as large fans, to prevent fluctuations in air movement that could negatively affect measurements Room temperature should be measured consistently and included in the test results, ensuring that the temperature sensor is shielded from direct optical radiation from any source.
In addition, since stray light could influence the measurement results, background lighting should be held to the lowest possible level during all measurements.
Photobrightening and photobleaching
Luminescent nanoparticles can display two contrasting behaviors when exposed to high-intensity excitation sources: photobrightening, where their emission efficiency increases, and photobleaching, where their emission efficiency decreases.
Photobrightening can be reversible, returning to its original efficiency once the excitation source is removed, or irreversible, while photobleaching is typically irreversible due to material degradation Both phenomena can distort efficiency measurements, necessitating careful monitoring of the sample's light exposure history It is crucial to minimize the excitation power and exposure time applied to the sample to reduce these effects without compromising the signal-to-noise ratio.
Luminescence from contaminants at Illumination wavelengths < 380 nm
Airborne contaminants such as smoke, hydrocarbons and fabric lint can accumulate in an integrating sphere over time These contaminants can fluoresce under UV irradiation
Fluorescence properties of integrating spheres must be characterized, especially when using UV excitation sources, as materials with wavelengths less than 380 nm can attenuate both excitation and emission signals Additionally, some reflective coatings may produce intrinsic parasitic emissions that cleaning cannot eliminate, leading to amplified fluorescence due to multiple internal reflections.
Procedures are available for correcting for this stray luminescence [1,2] 1
Industrial hygiene
Currently, there is limited knowledge regarding the environmental, health, and safety impacts of nanomaterials, leaving the effects of human exposure largely unknown International exposure standards are being developed to address these concerns In the meantime, it is essential to adhere to prudent laboratory practices to reduce exposure to nanomaterials For safe handling guidelines, it is advisable to consult available information and recommendations.
NOTE One reference is U.S National Institute for Occupational Safety and Health publication 2009-125 [3]
5 Measurement of relative quantum efficiency of nanomaterials
General
Relative measurements of quantum efficiency utilize standard reference materials with well-defined properties Numerous references detail the instrumentation and setup for fluorescence measurements, highlighting the commonality of these methods A typical example involves using a fluorescent organic dye with known quantum efficiency to assess the quantum efficiency of a colloidal suspension of quantum dots To begin this process, a calibration curve is established over a specific spectral range using the fluorescent dye, allowing for the determination of a sample's quantum efficiency in relation to this curve.
Measurements of this type are typically performed on liquid-phase materials, as the fluorescent dye standards may be readily produced as liquid solutions of known concentrations.
Test equipment
Required supplies and test equipment
Test equipment for relative measurements of quantum efficiency shall include the following:
In this article, we focus on standard fluorescence quartz cuvettes with a path length of 10 mm When utilizing cuvettes of varying sizes, it is essential to make the necessary adjustments to the solution volumes to ensure accurate measurements.
To ensure accurate quantum efficiency measurements, it is essential to thoroughly clean cuvettes, as incomplete cleaning can leave residues that negatively affect results A recommended practice is to acid-wash all quartz cuvettes before use to effectively remove any residual quantum dots.
– spectrophotometer with diffuse transmittance capability that measures absorption over the spectral region of interest (typically the ultraviolet and visible (UV-Vis) regions)
Wavelength calibration of the spectrophotometer should be verified at least annually using a light source of well-characterized emission wavelengths, such as a mercury argon calibration source;
– fluorescence spectrophotometer capable of producing excitation radiation in the spectral region of interest (typically UV-Vis) and measuring the excitation and emitted radiation
Additional information on the setup and calibration of fluorescence spectrophotometers
The excitation radiation is usually generated by a monochromated discharge lamp, which features an adjustable slit to control the peak full-width-at-half-maximum (FWHM) The emitted radiation from the sample goes through additional optics, including an emission slit and monochromator, before reaching the detector, such as a photomultiplier tube To ensure accurate measurements, a calibration file for the spectral response of both the emission monochromator and detector is essential, which can be sourced from the instrument manufacturer or created using a calibrated light source.
The UV-Vis spectrophotometer should be configured to scan the spectral range of 300 nm to 800 nm To ensure an optimal signal-to-noise ratio, acquisition parameters must be fine-tuned, with the minimum absorbance set at -0.05 and the maximum absorbance at 1.00.
When measuring sample fluorescence using a fluorescence spectrophotometer, it is essential to select an appropriate excitation wavelength along with the start and end wavelengths for collecting emission spectra To reduce re-absorption of fluorescence, it is important to minimize the overlap between the red edge of the excitation spectrum and the blue edge of the emission spectrum Research indicates that maintaining an optical density (OD) of less than 0.05 in the overlap region is crucial to mitigate re-absorption and inner-filter effects.
The slit width on both the excitation and emission monochromators should typically be set to the same value, balancing signal intensity and peak resolution It is advisable to use the minimum slit width that maintains an acceptable signal-to-noise ratio Additionally, the spectral bandpass conditions must remain consistent for both sample and reference measurements, and other spectrophotometer settings, such as the photomultiplier tube (PMT) voltage, should also be identical for both materials.
Fluorescence measurements are usually conducted over a more limited spectral range compared to absorbance measurements, allowing for the programming of various pre-determined acquisition protocols in many instruments This article presents representative values from three methods: the "Green QY method," the "Red QY method – high QY," and the "Red QY method – low QY."
Table 1 In this example, QY stands for quantum yield Different methods (and associated fluorescent material standards) would be required for other spectral regions of interest
Table 1 – Example fluorescence methods for relative measurements
Green QY method Red QY method – high QY Red QY method – low QY
Start collection 470 nm 545 nm 540 nm
End collection 700 nm 800 nm 850 nm
Excitation slit 2,5 nm 2,5 nm 2,5 nm
Emission slit 2,5 nm 2,5 nm 2,5 nm
PMT detector voltage Medium Medium High
Calibration
Fluorescent materials with well-defined quantum efficiencies are essential as calibration standards for relative measurements of solutions When selecting a reference material, it is crucial that its excitation wavelength closely matches the expected excitation wavelength of the sample for the intended application Additionally, the quantum efficiency of the reference material should be equal to or greater than the anticipated value for the samples being tested In solid-state lighting (SSL) applications, the excitation wavelength typically falls within the range of 440 nm.
For accurate calibration of luminescent nanomaterials, it is essential to use an excitation wavelength of 470 nm, although other wavelengths may also be applicable The emission wavelength of the standard should closely match that of the sample A comprehensive list of potential reference materials is available in reference [8] and other sources Depending on the spectral region of interest, multiple fluorescent materials may be necessary for precise calibration Table 2 provides examples of quantum efficiency calibration standards for relative measurements, while additional calibration standards for other wavelengths can be found in reference [8].
Table 2 – Suggested calibration standards for relative quantum efficiency measurements of luminescent nanoparticle solutions
Fluorescent standard Solvent Excitation wavelength
Method used Quantum efficiency Reference
Weigh approximately 2 mg of the fluorescent calibration standard using a microbalance and place it in a 20 ml vial Dissolve the dye in 10 ml of the suitable solvent to prepare a concentrated stock solution of the calibration standard.
Remove 2 ml of the concentrated stock solution via syringe and place it into a 20 ml vial
Dilute the solution with an additional 8 ml of original solvent to create the dilute stock solution of the calibration standard
Remove 2,5 ml of solvent and place into a quartz cuvette Then run a baseline in the UV-Vis spectrophotometer
Using a microsyringe, add 100 àL of the calibration standard dilute stock solution to the cuvette and mix well
Take an absorbance measurement on the UV-Vis spectrophotometer and note the optical density (OD) at the excitation wavelength of choice
To determine the OD/àL stock solution ratio, divide the recorded optical density (OD) by 100 Adjust the concentration of the quartz cuvette solution until the OD at the excitation wavelength reaches 0.05, as illustrated in Figure 1.
Check this calculation by taking an absorbance measurement on the spectrophotometer
To ensure accurate measurements with the fluorescence spectrophotometer, follow the methods outlined in section 5.2.2.2 It is crucial to verify that the maximum concentration does not lead to a non-linear response, such as saturation, in the detector If saturation is detected, adjustments to the method parameters are necessary.
Figure 1 – Sample absorbance spectrum of cresyl violet – example calculations
Experimental procedure
Remove 2,5 ml of the solvent used for the standard reference material and place into a quartz cuvette Measure a baseline in the UV-Vis spectrophotometer
To determine the absorbance levels of 0.001, 0.003, 0.005, 0.01, 0.03, and 0.05 at the selected excitation wavelength, calculate the required volume of the standard dilute stock solution using the OD/àL stock solution ratio outlined in section 5.3.3.2 These absorbance values, along with their corresponding integrated emission intensities, will facilitate the creation of a calibration curve essential for assessing relative quantum efficiency.
NOTE The OD (i.e., absorbance) should not exceed 0,05 due to non-linear behaviour in the Beer-Lambert law, including re-absorption effects, associated with higher concentrations
Add the appropriate initial amount of dilute stock solution to the solvent-containing cuvette and take measurements in the UV-Vis spectrophotometer and fluorescence spectrophotometer
Add the appropriate second amount of stock solution to the cuvette
Repeat the measurements for each additional amount of the standard stock solution added
100 uL dilute stock solution, 2,5 mL methanol Excitation wavelength of choice: 530 nm
OD / uL ratio = 0,05 / 100 uL = 0,0005 / 1 uL
For accurate data analysis, open all emission spectra from the fluorescence spectrophotometer and multiply each by the “Correction Lamp” file from a calibrated optical radiation source This file reflects the spectrophotometer's detector response across different wavelengths and should be provided by the manufacturer or created using a tungsten halogen calibration source Alternative calibration methods, such as using calibrated reference detectors, can also be employed, necessitating the application of suitable calibration corrections.
Save each corrected spectrum into a spreadsheet format Calculate the total integrated peak area over the corrected emission peak
5.4.1.4 Data input and calibration curve plot
NOTE The format described is based on the use of Microsoft Excel®2
Using spreadsheet software, assign columns for absorbance (at the appropriate excitation wavelength) and for integrated fluorescence intensity Input all of the data collected from the measurements
Plot the data for the fluorescent reference standard with absorbance on the x-axis and integrated fluorescence intensity on the y-axis
To create a linear regression trend line with the intercept set to zero, plot the data and display the corresponding equation and R-squared value on the chart This calibration curve is linked to a specific fluorescent reference standard, and it is essential to generate similar plots for each standard.
The slope of the plot is essential for assessing the relative quantum efficiency of the luminescent nanoparticle solution By analyzing historical slope values for a specific standard, we can calculate mean values and standard deviations It is crucial to compare the current linear regression slope with the historical mean and check if it deviates by more than three standard deviations If the new reading exceeds this threshold, the calibration procedure must be repeated.
To accurately compare slope values of a calibration standard reference material, it is essential that the data collection methods are consistent This consistency must encompass factors such as excitation wavelength, slit width, collection start and end points, PMT detector voltage, and the quantum efficiency of the fluorescent reference standard employed.
A deviation exceeding three standard deviations indicates that the spectrophotometer is out of control, necessitating further investigation into the cause and the implementation of corrective actions.
Luminescent nanoparticle sample − Experimental measurements
Remove 2,5 ml of solvent used for the luminescent nanoparticle sample and place into a quartz cuvette Measure a baseline in the UV-Vis spectrophotometer
5.4.2.2 Sample measurement – Single point measurement
Measure the absorbance of the nanoparticle sample using the UV-Vis spectrophotometer If the sample absorbance at the excitation wavelength of interest exceeds 0,05, dilute with
Microsoft Excel® is a registered trademark of Microsoft Corporation This information is provided for user convenience and does not imply IEC endorsement of the product Equivalent products may be utilized if they yield comparable results For optimal results, additional solvent should be added until the absorbance is below 0.05, with the ideal optical density (OD) range for the sample being between 0.03 and 0.05.
To analyze the samples, first record the absorbance using a UV-Vis spectrophotometer and then measure the emission spectrum with a fluorescence spectrophotometer Ensure to apply all necessary data corrections and calculate the total integrated peak area of the corrected emission peak If required, repeat this procedure for additional samples.
5.4.2.3 Quantum efficiency calculation – Single point measurement
Quantum efficiencies of luminescent nanoparticles are calculated according to the format in
Table 3 – Spreadsheet format for quantum efficiency data comparisons
Absorbance of sample at nm Corrected integrated fluorescence intensity Slope of standard reference material (e.g., dye)
Input the data for columns A and B For column C, enter in the slope calculated from the standard fluorescent dye calibration curve measurements (see 5.4.1.4) For column D, enter the following equation:
D i = (QE Standard * B i * (Ref Index Sample) 2 ) / (A i * C i * (Ref Index Standard) 2 ) where:
A i , B i , C i , D i = entries in the spreadsheet cells for columns A, B, C, D, respectively, and row number i (i.e., i = 1,2 3, etc.)
QE Standard = quantum efficiency of the calibration standard reference material used
Ref Index Sample = refractive index of the solvent used with the sample
Ref Index Standard = refractive index of the solvent used to dissolve the standard dye
Test the quantum efficiency spreadsheet by measuring against other fluorescent standards of known quantum efficiency calibration standard dyes
The data collection methods for both standard dye and luminescent nanoparticles will be consistent, utilizing the same excitation wavelength, slit width, collection start and end points, and PMT detector voltage.
5.4.2.4 Sample measurement – Full linear regression
To analyze the samples, repeat the procedure outlined in section 5.4.2.2 using an optical density (OD) range similar to that of the calibration standard data, specifically 0.001, 0.003, 0.005, 0.01, 0.03, and 0.05 OD For each sample, measure the absorbance with a UV-Vis spectrophotometer and obtain the emission spectrum using a fluorescence spectrophotometer Ensure to apply all necessary data corrections as described in section 5.4.1.3, and calculate the total integrated peak area of the corrected emission peak according to section 5.4.1.4 If needed, repeat this process for additional nanoparticle samples.
5.4.2.5 Quantum efficiency calculation – Full linear regression
Utilize spreadsheet software to create columns for absorbance at the designated excitation wavelength and for the integrated fluorescence signal, usually measured in counts per second or similar units Enter all collected measurement data into the spreadsheet.
NOTE The format described is based on the use of Microsoft Excel®
Plot the nanoparticle data with absorbance on the x-axis and integrated fluorescence intensity on the y-axis
To create a linear regression trend line from the data, set the intercept to zero, and display both the equation and R-squared value on the chart The slope of this plot will be utilized to calculate the relative quantum efficiency of the luminescent nanoparticle solution, as detailed in Table 4.
Table 4 – Spreadsheet format for quantum efficiency data comparisons
(e.g., QDs) Slope of standard reference material (e.g., dye) Quantum efficiency of nanoparticle solution
Input the data for columns A and B For column C, enter the following equation:
C,= QE Standard * (A i / B i ) * (Ref Index Sample) 2 / (Ref Index Standard) 2 where:
A i , B i , C, = entries in the spreadsheet cells for columns A, B, C, respectively, and row number i (i.e., i = 1,2 3, etc.)
QE standard = quantum efficiency of the calibration standard reference material used
Ref Index Sample = refractive index of the solvent used with the sample
Ref Index Standard = refractive index of the solvent used to dissolve the standard dye
Both standard dye and luminescent nanoparticle data collection will utilize the same methods, ensuring consistency in excitation wavelength, slit width, collection start and end points, and PMT detector voltage.
6 Measurement of absolute quantum efficiency of nanomaterials
General
Absolute measurement techniques assess quantum efficiency based on fundamental standards of mass, time, and distance Calibration in these measurements often utilizes reference standards that are traceable to base or derived SI units, such as NIST-traceable standard reference materials This measurement approach is applicable to both solid and liquid samples.
This document outlines two methods for measuring the absolute quantum efficiency of luminescent nanomaterials: the collimated incident light method (6.5.1) and the diffuse incident light method (6.5.2) A comparative analysis of these methods is presented in Table 5.
Table 5 – Comparison of methods for measuring the absolute quantum efficiency of luminescent nanoparticles
Collimated incident light Diffuse incident light
Required equipment See Figure 2 and 6.2 See Figure 2 and 6.2
Number of measurements Three Two
Values used in calculations Photon flux (1/s) Spectral radiant flux (W/nm)
Absolute measurements of the quantum efficiency of both liquid and solid samples can be performed using either a collimated incident light method or a diffuse incident light method
The equipment for both methods is very similar and the experimental set-ups are given in
Collimated incident light method Diffuse incident light method
Figure 2 – Schematic of the test equipment configuration for both the collimated incident light and diffuse incident light methods
Test equipment
When selecting an integrating sphere for testing, it is essential to choose one with an interior coated in a material that has over 95% diffuse reflectance The sphere's diameter should be at least three times larger than the longest dimension of the sample to ensure uniform light diffusion While a larger sphere is generally preferable, excessively large spheres may reduce the radiance reaching the detector Typically, a sphere with a diameter of 10 cm or more is adequate Additionally, the sphere should feature a removable sample holder and at least two equatorial ports, with the total area of the ports not exceeding 5% of the sphere's surface area For detailed guidelines on sphere diameters and port sizing, refer to reference [14].
The sample holder, coated with high diffuse reflectance material, securely holds the sample within the sphere In the collimated incident light method, it is essential for the sample holder to enable movement of the sample into and out of the light beam Conversely, the diffuse incident light method does not require such movement, as the irradiation is provided solely by a diffuse light source.
– Detector and light source ports meeting the following requirements:
The detector port (Port 1 in Figure 2) must include a light baffle that effectively prevents direct illumination of the detector from the light source Additionally, this port is equipped with an adapter for enhanced functionality.
A typical SMA connector interfaces with a fiber optic cable outside the sphere Positioned in front of the adapter is a high transmittance light diffuser featuring near-Lambertian characteristics, which guarantees effective wide-angle light collection.
The excitation light source port (Port 2 in figure 2) shall consist of a small aperture
(typically less than 1,27 cm) through which light from the excitation source is introduced
The collimated incident light method involves directly introducing the light source through the aperture without any modifications In contrast, the diffuse incident light method utilizes a high transmittance light diffuser with near-Lambertian properties, which is positioned across the aperture to effectively diffuse the excitation light entering the sphere.
A spectroradiometer is designed to precisely measure radiation intensity across specific wavelengths, generally ranging from 350 nm to 900 nm It features a grating that is blazed to ensure optimal detection within this targeted wavelength range.
To enhance collection efficiency and minimize stray light, optical elements like lenses and filters can be integrated into the spectroradiometer The device is linked to the integrating sphere at Port 1 via an external fiber optic cable Additionally, the spectroradiometer undergoes annual wavelength calibration verification using a well-characterized light source, such as a mercury argon calibration source.
Fibre optic cables are the preferred method for connecting a spectroradiometer to an integrating sphere However, when the signal-to-noise ratio is adequate, focusing optics with an f-number compatible with the spectroradiometer's monochromator can be utilized at Port 1 as an alternative to fibre optic cables It is essential to clearly document this modification in the test report.
– Narrow-band light source chosen from one of the following:
Laser: the desired source with easiest control of wavelength, spectral width (FWHM) and beam shape (collimated narrow beam) This source allows for convenient delivery of light
The collimated incidence method allows for high precision control of output stability through temperature and current adjustments The output can be either pulsed or continuous wave (cw), with pulsing often introducing noise from pulse-to-pulse intensity variations The degree of this noise is influenced by the laser type and the pulse creation method, which can be modulated to enhance the signal-to-noise ratio.
LEDs are an affordable light source that offers advantages such as wavelength selection and intensity, although they present challenges in delivering collimated light They are particularly effective for diffuse incidence methods To ensure reliable operation, it is essential to maintain proper temperature and current/voltage control Additionally, it is important to consider the width of the LED emission spectra to avoid overlap with the emission spectra of the sample being analyzed.
– Monochromatic discharge lamps: still a widespread and well-tested reliable option
The use of this light source can be complex due to its relatively low intensity per spectral interval, which is affected by the narrow bandpass that decreases the monochromator's throughput Additionally, the finite f-number of the instrument and the lack of collimation further diminish intensity This source is more appropriate for diffuse incidence light methods, but it may also encounter issues with output stability in both pulsed and continuous wave (cw) modes.
A calibrated light source, such as a spectral radiant flux or irradiance standard, is essential for use in an integrating sphere The output of this light source is established using traceable standards, accompanied by a calibration file Calibration is performed following the specified procedure outlined in section 6.3.
– Computer for collection and analyzing data
Optional accessories for the integrating sphere include short pass optical filters to refine the peak shape of the excitation source, adapters for compatibility with various light sources, and blanking plugs for sealing unused ports.
NOTE See Annex A for general considerations regarding sample heating and excitation density.
Calibration
The calibration process for absolute measurements of luminescent nanoparticles necessitates the simultaneous calibration of all measurement equipment as a cohesive system This encompasses the integrating sphere, sample holder, spectroradiometer, fiber optic cable, and additional components.
In order to accurately calibrate the test equipment, a calibrated light source shall be used
A calibrated light source, commonly a tungsten filament, has a correlated color temperature of approximately 3,000 K and operates within the wavelength range of 350 nm to 2,000 nm For wavelengths beyond this range, alternative standards like a deuterium lamp should be utilized.
(200 nm to 400 nm) The calibrated light source can be introduced into the sphere either through the aperture in Port 2 or through a special fixture attached to Port 2
A calibration file containing the spectral radiant flux of the standard (typically in W/nm) measured relative to a traceable standard is used to calibrate the test equipment as a whole
The calibrated light source is introduced into the test equipment and the spectral radiant flux
The spectroradiometer measures the observed spectral flux, Φ obs (λ), with an acquisition time optimized to achieve the highest practical value without inducing saturation or non-linear detector responses This approach guarantees maximum sensitivity for both calibration and sample measurements.
The observed spectral radiant flux (\$Φ_{obs}(λ)\$) is analyzed against the calibration file at each wavelength to establish a correction factor (\$C(λ)\$) The absolute spectral radiant flux (\$Φ_{ab}(λ)\$) for the standard at each wavelength is calculated by multiplying the correction factor by the observed spectral radiant flux, expressed as \$Φ_{ab}(λ) = C(λ) \times Φ_{obs}(λ)\$.
The correction factors (C(λ)) are used to correct values measured under identical experimental conditions for various samples.
Sample preparation
The absolute measurement of quantum efficiency can be performed on either solution or solid-state samples The sample preparation requirements for each are given in this subclause
Using a microbalance, weigh out a desired amount of the sample and dilute with an appropriate solvent to create a concentrated stock solution
Remove 2 ml of the concentrated stock solution via syringe and place it into a 20 ml vial
Dilute the solution with an additional 8 ml of original solvent
For accurate absolute measurements in integrating spheres, it is essential to utilize the smallest feasible solution volume Additionally, specifying the concentration of the luminescent material is crucial to ensure the measurement is contextualized appropriately.
Solid state samples are created by combining quantum dots or other luminescent materials with a chosen matrix, like silicone or epoxy, in a specific weight ratio, followed by solidification under controlled conditions To protect the luminescent nanoparticles from environmental damage, the mixture can be encased in transparent packaging The sample volume should be optimized for accurate quantum efficiency measurements, and if needed, can be diluted with nonfluorescent materials such as pressed barium sulfate or sintered polytetrafluoroethylene powder The appropriate sample volume and dilution depend on the concentration of luminescent nanoparticles, the detector's dynamic range, and the size of the integrating sphere used.
One possible configuration for measuring solid samples is to place a drop of uncured QD or other luminescent nanoparticle mix in a transparent organic matrix between two glass slides
Spacers can be used to keep glass slides apart during subsequent cure.
Test procedure
The collimated incident light method provides an absolute measurement of the quantum efficiency of luminescent nanomaterials in both liquid and solid phases This technique involves introducing a collimated light beam into an integrating sphere, serving as the main excitation source For more comprehensive information on this testing method, please refer to additional resources.
After calibrating the test equipment, the calibrated light source is substituted with the chosen excitation light source, such as a laser, LED, or monochromated discharge lamp It is essential that the light source is collimated and directed into the integrating sphere through the aperture at Port 2 To ensure accurate measurements, the beam divergence should be minimized, making the beam size entering the integrating sphere and hitting the sample at least 50% smaller than the sample itself.
In the collimated incident light method, the integrating sphere's sample holder must allow for the sample to be positioned in and out of the direct path of the incident beam, as illustrated in Figure 2 This technique involves taking three distinct measurements for each sample.
Experiment A Background measurement with no sample in the integrating sphere
In Experiment B, the sample is positioned within the integrating sphere but is shifted away from the collimated incident light This adjustment ensures that the incident light beam initially interacts with the walls of the integrating sphere, allowing only diffuse radiation to reach the sample.
In Experiment C, the sample is placed within the integrating sphere to ensure that the collimated incident light beam directly illuminates it This positioning allows any reflected light to hit the walls of the integrating sphere, preventing it from re-entering through the entrance.
When using the collimated incident light method, it is crucial to conduct all calculations using values measured in units proportional to photons per second Additionally, ensure that the spectroradiometer connected to the integrating sphere is calibrated for spectral radiant flux in appropriate units.
W/nm, these values can be readily converted into a photon flux (in units of 1/second) The procedure for this conversion is as follows:
The radiant energy of a photon is given by the equation
Q = photon energy in joules (J) h = Planck’s constant (6.626 × 10 -34 joules second (J s)) c = speed of light (3 × 10 8 metres/second (m/s)) λ = wavelength (in metres)
If the wavelength is expressed in nanometres (nm), this equation reduces to
When multiple photons exist at a specific wavelength, the total radiant energy at that wavelength can be calculated by multiplying the number of photons (n) by the energy of each photon.
Q total = nQ λ = n(1,99*10 -16 /λ) Where: n = number of photons λ = wavelength (in nm)
From this equation, a relationship for the number of photons can be derived: n = (5,03*10 15 )Q total λ
Photon flux, measured in units of 1/s, is derived by replacing the photon radiant flux (Φ), expressed in watts or joules per second, with photon energy Consequently, the formula for photon flux is established.
Where: Φ = photon radiant flux (W) λ = wavelength (nm)
Consequently, the product of wavelength and photon radiant flux will provide a value proportional to the number of photons per second
The spectral power distribution can be represented graphically, where the y-axis indicates the number of photons per second across a specific wavelength range This is achieved by multiplying the wavelength by the spectral radiant flux at that particular wavelength An illustrative example is provided.
Figure 3 – Sample spectrum for collimated incident light method
In Experiment A, the background photon flux is measured without a sample in the integrating sphere, primarily due to stray light from parasitic scattering within the sphere This instrument background signal must be subtracted from all subsequent measurements to ensure accurate results.
Place the sample in the integrating sphere directly in the path of the incident beam
(Experiment C) Since the light beam is collimated, it will strike the sample at a normal incidence angle when the sample is placed directly into the beam (Experiment C) A fraction,
In this incident light method, the sample absorbs some light while the rest is either transmitted or reflected The transmitted and reflected light then strikes the walls of the integrating sphere, where it is diffusely scattered back toward the sample Additionally, any light emitted by the sample is also diffusely scattered within the integrating sphere Consequently, the sample primarily receives radiation from the collimated light source at an angle of incidence close to 0 degrees, along with additional radiation from the diffusely scattered light at various angles.
When a sample is positioned in the integrating sphere but not directly in the path of the incident collimated light, it will only receive illumination from diffusely scattered radiation at various angles of incidence.
In Experiment B, a portion of the scattered light, denoted as à, is absorbed by the sample, while the rest is either transmitted or reflected The transmitted and reflected light subsequently interacts with the walls of the integrating sphere, where it is diffusely scattered back toward the sample.
Any light emitted by the sample will also be diffusely scattered by the integrating sphere
The analysis of the spectrum from the three measurements involves identifying the source and photoluminescent emission peaks By subtracting the instrument background, the area under each peak is calculated through integration, yielding a value that is proportional to the number of photons emitted per second within a specific wavelength range The area under the source peak, denoted as L, indicates the amount of light emitted by the source that is not absorbed, while the area under the emission peak, referred to as P, measures the emitted light Additional corrections, such as stray light compensation, are also applied to enhance the accuracy of the results.
P absorption [17] may also be applied At a minimum, the analyzed spectrum will contain the following values:
L a – source peak area with no sample in the integrating sphere (Experiment A)
L b – source peak area with sample in the integrating sphere but diffusely illuminated only
L c – source peak area with sample in the integrating sphere and illuminated by collimated incident light and diffusely scattered light (Experiment C)
P b – photoluminescence emission peak area with sample in the integrating sphere but diffusely illuminated only (Experiment B)
P c – photoluminescence emission peak area with sample in the integrating sphere and illuminated by collimated incident light and diffusely scattered light (Experiment C)
All values of L and P shall be converted to units proportional to photons per second using the procedures described above in 6.5.1
From these parameters, the fraction of incident light that is absorbed (A) and quantum efficiency (η) can be calculated using the formulas below (method described in more detail elsewhere [15]):
This expression can also be written as:
The diffuse incident light method enables the accurate measurement of quantum efficiency in luminescent nanomaterials, applicable to both liquid and solid-phase materials This technique provides an absolute measurement of quantum efficiency by utilizing a diffuse light beam that excites the samples from all angles of incidence.
After calibration of the test equipment, the calibrated light source is replaced with a light source (e.g., laser, LED, or monochromated discharge lamp) and a light diffuser as shown in
Overview
The impact of temperature on quantum efficiency (TQE) is crucial for accurate QE measurements Sample heating is influenced by several factors, including the energy gap between excitation and emission spectra, known as Stokes loss, as well as the unknown initial quantum efficiency of the material in the absence of incident radiation.
Lower quantum efficiency (QE) leads to increased heating of the sample for the same excitation density This effect is influenced by the host matrix containing the luminescent nanomaterials, the thermal coupling between the sample and the experimental setup, and the heat dissipation capabilities of the setup When characterizing QE as a function of excitation density, this phenomenon becomes evident While various hardware solutions can mitigate this issue, one effective approach is to utilize a modulated light source, such as a diode laser, to address the thermal effects on total quantum efficiency (TQE).
Addressing TQE
Diode lasers are highly regarded for their efficient optical beam management, stable output, and the ability to independently adjust pulse width and duty cycle The pulse width is primarily constrained by the rise and decay times of luminescence from the sample, which must be optimized to ensure saturation of emission For instance, a YAG:Ce sample exhibits a luminescence rise time of approximately 0.25 µs It's important to recognize that these rise and fall times vary with excitation density and measurement setup, necessitating checks across different excitation levels To prevent heating, very short pulses or a low duty cycle should be employed Overall, the maximum heat deposited can be expressed as \(\Delta Q = \Delta t \times \text{(other factors)}\).
The power (P) applied during a pulse increases the temperature of a material based on its specific heat capacity (C) and mass (m), as described by the equation \(\Delta T = \frac{\Delta Q}{mC}\) This temperature rise can vary from a fraction of a degree to several degrees above the ambient temperature with each heating pulse.
Figure A.1 – Example of transient behaviour of luminescent material (YAG:Ce) under pulsed excitation
S ignal , a rbi tr ar y uni ts
In this scenario, the excitation pulse width exceeds the saturation time for the luminescence signal However, this may not accurately represent the conditions in different user applications Consequently, users might intentionally introduce heating without fully understanding its specific effects, relying instead on measuring the overall quantum efficiency (QE) anticipated from the material under their defined conditions.
The duty cycle's boundary conditions are determined by the detector's dynamic response in the QE setup and the heating of the sample A typical curve in Figure A.2 demonstrates the calculated quantum efficiency (QE) as a function of average excitation density This average excitation density can be adjusted by independently varying the pulse width and the duty cycle.
Quantum efficiency (QE) is assessed through a specific test procedure, as outlined in section 6.5 Various duty cycle and pulse width combinations can create similar curves for different peak laser power levels In Figure A.2, the dashed line marks a region where heating begins, typically leading to a shift in the emission spectra of the quantum dot (QD) sample Consequently, QE measurements should be conducted at excitation powers below this threshold Additionally, at high excitation levels, it is crucial to ensure that the detector operates within its linear range, while also accounting for physical effects that cause inherent sub-linearity in the light output of most materials at elevated incident flux values Standardized methods for evaluating the linearity of fluorescence measurement instruments are discussed in other sources.
Figure A.2 – Schematic diagram of variation of normalised QE with average excitation power and the preferred range of input power (indicated by vertical lines)
An alternative to pulsed diode lasers is a temperature-stabilized continuous wave (cw) laser equipped with an electronically controlled frequency modulation (chopping) wheel This combination of hardware enables access to longer pulse widths and lower frequencies, ranging from sub-Hz to kHz, although not entirely independently.
The advantage is improved signal-to-noise ratio compared to cw alone and riddance of pulse generation noise evident in non-solid state laser sources
The physical mechanisms influencing absorption behavior are detailed in the literature and are contingent upon the nature of absorption, which affects the excitation volume High concentrations of microscopic oscillators or elevated absorption coefficients lead to shallow absorption depths and increased excitation densities Therefore, measurement conditions should be tailored based on whether the focus is on the material's intrinsic properties or application-specific characteristics To achieve optimal intrinsic performance, it is advisable to utilize the lowest intensity possible while maintaining an adequate signal-to-noise ratio during data acquisition.
[1] P.-S SHAW et al Measurement of the ultraviolet-induced fluorescence yield from integrating spheres Metrologia, 2009, 46, S191-S196
[2] R D SAUNDERS et al Spectral irradiance measurements: effect of uv-produced fluorescence in integrating spheres Applied Optics, 1976, Vol 15, Iss 4, pp 827–828
[3] U.S Department of Health and Human Services, Approaches to Safe Nanotechnology:
Managing the Health and Safety Concerns Associated with Engineering Nanomaterials
[4] P.C DEROSE and U.RESCH-GENGER Recommendations for Fluorescence Instrument
Qualification: The New ASTM Standard Guide Analytical Chemistry, 2010, volume 82 pp 2129 – 2133
[5] IUPAC Project no 2004 021 1 300 Fluorometry Task Group
[6] ASTM E 578-07, Standard test method for linearity of fluorescence measuring systems
[7] ASTM E 388-04, Standard test method for wavelength accuracy and spectral bandwidth of fluorescence spectrometers
[8] D.F EATON Reference materials for fluorescence measurement Pure and Applied
[9] ASTM E2719-09, Standard guide for fluorescence – Instrument calibration and qualification
[10] R.F KUBIN and A.N FLETCHER Fluorescence quantum yields of some rhodamine dyes J Luminescence, 1983, 27, 455
[11] M DOUGLAS et al Absolute luminescence yield of cresyl violet A standard for the red
[12] T KARSTENS and K KOBS Rhodamine B and Rhodamine 101 as reference substances for fluorescence quantum yield measurements J Phys Chem., 1980, 84,
[13] A.M BROUWER Standards for photoluminescence quantum yield measurements in solution (IUPAC Technical Report) Pure and Applied Chemistry, 2011, 83, 2213
[14] Labsphere, A guide to integrating sphere theory and applications Available at www
[15] J.C DE MELLO, H.F WITTMANN, R.H FRIEND An improved experimental determination of external photoluminescence quantum efficiency Advanced Materials,
[16] Y ZONG, S.W BROWN, B.C JOHNSON, K.R LYKKE, and Y OHNO Simple spectral stray light correction method for array spectroradiometers Applied Optics, 2006, 45,
[17] T.-S AHN, L AL-KAYSI, A.M MULLER, K.M WENTZ, and C.J BARDEEN., Self- absorption correction for solid-state photoluminescence quantum yields obtained from integrating sphere measurements Review of Scientific Instruments, 2007, 78 086105-1
[18] L.S ROHWER and J.E MARTIN Measuring the absolute quantum efficiency of luminescent materials,” Journal of Luminescence 115 (2005) 77
[19] ISO/IEC Guide 98-3:2008, Uncertainty of measurement – Part 3: Guide to the expression of uncertainty in measurement (GUM:1995)
[20] A BRIL, Physica 15, 1949, 361 (); A BRIL, Philips Technincal Rev 1950, 12 120
[21] D M DE LEEUW, G W ‘T HOOFT, J Lumin 28 (1983) 275 and references therein
[23] ISO TS 80004-2 _ 3 , Nanotechnologies – Vocabulary – Part 2: Nano-objects: nanoparticle, nanofibre and nanoplate
4 Notes générales sur les essais 40
4.3 Mise en évidence photo et décoloration 40
4.4 Luminescence de contaminants à des longueurs d'onde d'éclairement
5 Mesure du rendement quantique relatif des nanomatériaux 41
Matériel et équipement d'essai exigés 41
Norme d'étalonnage – réalisation d'essais de mesure 44
Norme d'étalonnage – réalisation de mesures expérimentales 45
Echantillon de nanoparticules luminescentes – Mesures
6 Mesure du rendement quantique absolu des nanomatériaux 48
Méthode de lumière incidente collimatée 52
Méthode de lumière incidente diffuse 56
Annexe A (informative) Extinction thermique du rendement quantique, utilisation de la modulation lumineuse en vue d'éviter l'échauffement de l'échantillon et d'établir des conditions de mesure optimales 60
Figure 1 – Spectre d'absorbance d'un échantillon de violet de crésyl – exemples de calculs 44
Figure 2 – Représentation schématique de la configuration de l'équipement d'essai employé dans la méthode de lumière incidente collimatée et la méthode de lumière incidente diffuse 49
Figure 3 – Spectre de l'échantillon dans la méthode de lumière incidente collimatée 54
Figure 4 – Spectre de l'échantillon dans la méthode de lumière incidente diffuse 57
Figure A.1 – Exemple de comportement transitoire d'un matériau luminescent
(YAG:Ce) soumis à une excitation pulsée 61
Figure A.2 – Représentation graphique de la variation du QE normalisé avec la puissance d'excitation moyenne et de la gamme préférentielle de puissances d'entrée
(indiquée par des traits verticaux) 62
Tableau 1 – Exemples de méthodes de fluorescence destinées aux mesures relatives 43
Tableau 2 – Normes d'étalonnage suggérées pour les mesures du rendement quantique relatif des solutions de nanoparticules luminescentes 43
Tableau 3 – Feuille de calcul utilisée dans les comparaisons de données de rendement quantique 46
Tableau 4 – Feuille de calcul utilisée dans les comparaisons de données de rendement quantique 48
Tableau 5 – Comparaison des méthodes de mesure du rendement quantique absolu des nanoparticules luminescentes 49
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La Norme internationale IEC 62607-3-1 a été établie par le comité d'études 113 de l'IEC:
Normalisation dans le domaine des nanotechnologies relatives aux appareils et systèmes électriques et électroniques
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Nanofabrication – Caractéristiques de contrôle clé, peut être consultée sur le site web de l'IEC
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