A J Rijke
email: a.j.rijke@tue.nl

S Nijdam

M Haverlag

J F J Janssen

J J A M Van Der Mullen

G M W Kroesen

A calibrated integrating sphere setup to determine the infrared spectral radiant flux of high intensity discharge lamps

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Introduction

High intensity discharge lamps (HID lamps) are used when compact light sources are needed with high output levels and efficacies. This study deals with the experimental determination of the energy balance of HID lamps, i.e. the distribution of the total output energy of the lamp over different spectral regions is investigated. By determining and studying the energy balance of the lamp, the understanding of the energy flows inside the lamp can be improved. This can aid in the improvement of existing lamps and development of new ones. Since a long time the determination of the energy balance of HID lamps is an important aspect of HID lamp research. Studies and publications on HID lamps efficacy research and/or energy balances include those performed by Waymouth [START_REF] Waymouth | Electric Discharge lamps[END_REF], Elenbaas [START_REF] Elenbaas | Light Sources[END_REF], Jack and Koedam [START_REF] Jack | Energy balances for some high gas pressure discharge lamps[END_REF], De Groot and Van Vliet [START_REF] De Groot | The High-Pressures Sodium Lamp[END_REF] and Wharmby [START_REF] Wharmby | Energy balance of high-pressure sodium discharges under controlled vapour conditions[END_REF]. From these studies it can be concluded that infrared radiation is an important contribution to the energy balance of HID lamps. Infrared radiation, either in the form of radiation from the plasma or in the form of thermal radiation from the burner wall, can consume 50-75% of the total input power. The ALITE-II project's [START_REF] Gough | Program on Technology Innovation: Advanced Light Source Research: ALITE-II: Losses in High Intensity Discharge Light Sources 1012994[END_REF] primary focus was the infrared radiation of HID lamps; several papers regarding the infrared radiation from HID lamps have recently been published [START_REF] Smith | Infrared losses from a Na/Sc metal-halide high intensity discharge arc lamp[END_REF][START_REF] Lawler | Infrared continuum radiation from high and ultra-high pressure mercury lamps[END_REF][START_REF] Herd | Infrared continuum radiation from metal halide high intensity discharge lamps[END_REF]. In these papers the focus was mainly on the continuum radiation generated by the plasma.

As far as we know, other authors studying the infrared radiation of HID lamps have based their infrared work on side-on measurements, and/or did not measure the radiation beyond about 2.5 µm spectrally resolved [START_REF] Wharmby | Energy balance of high-pressure sodium discharges under controlled vapour conditions[END_REF][START_REF] Gough | Program on Technology Innovation: Advanced Light Source Research: ALITE-II: Losses in High Intensity Discharge Light Sources 1012994[END_REF][START_REF] Smith | Infrared losses from a Na/Sc metal-halide high intensity discharge arc lamp[END_REF]. We will use spectrally resolved absolutely calibrated integrating sphere measurements to determine the infrared contributions to the spectrum up to 10 µm.

An integrating sphere is a hollow sphere with a diffuse high reflectivity coating on its inner surface. See section 2.2 for a description of the integrating sphere used in this paper. The light source is placed inside the sphere. Due to the many diffuse reflections on the inner surface, light leaving a hole in the sphere is a well-mixed measure for the total (radiant) flux of the source. However, small changes in the spectral reflectivity of the inner surface can cause large differences in the signal [START_REF] Bergman | The Art and Science of Lamp Photometry[END_REF]. Therefore, this measurement system has to be carefully calibrated before absolute measurements can be performed. In the visible range this is done using standard lamps, but for the infrared this is more of an issue because existing calibration sources are either not designed for the use in the (mid-)infrared, or are only suitable for side-on measurements.

A calibration procedure has been developed to perform the calibration of the integrating sphere in the infrared. In this calibration procedure a platinum strip inside the integrating sphere is heated by means of Ohmic dissipation. The temperature profile and spectral radiant flux of the platinum strip are calculated using a model, see section 3.1. This flux is integrated over the area of the strip and this integrated flux is used as the reference signal for the calibration. The integrating sphere has been calibrated in the infrared up to 10 µm.

Absolutely calibrated integrating sphere measurements have been performed in the infrared on a Philips CDM-T 70W/830 HID lamp. For this lamp, the output part of the energy balance is mapped out in detail, see section 4.1. The energy in the wavelength range above 10 µm is determined by assuming it is following the black body radiation curve for a constant emissivity and a temperature of 1300K. To correct for the radiating surface and the emissivity, this black body curve is scaled to connect to the measured infrared spectrum at 10 µm. The amount of visible radiation in the energy balance was acquired from integrating sphere measurements at Philips Lighting Optical Calibration Laboratory (OCM) in Eindhoven.

The HID lamp used in this paper has an efficacy of 93 lm/W. Typical values for HID lamp efficacies are between 60 and 120 lm/W, this is almost one order of magnitude higher than the typical efficacy of an incandescent lamp. For linear fluorescent tubes, typical values are between 80 and 100 lm/W. For LED lamps, typical values are 50 to 100 lm/W (all under operating conditions). To be able to make a comparison considering the total system, also the efficiency with which the (electronic) lamp drivers operate is important. For HID lamp drivers, typical efficiencies are in the range of 90-95%, for linear fluorescent tubes typical values are around 90% and for LED the driver efficiencies are typically between 80 and 90%.

Our setup and the details of the calibration procedure are discussed in sections 2 and 3 respectively. The infrared measurements are discussed in more detail in section 4. The output part of the energy balance of this lamp is presented in section 4.1.

Experiment

The energy balance

An example of a typical representation of an energy balance is given in figure 1 (after [START_REF] Schafer | Energy Efficient Light Sources[END_REF]). For a more thorough discussion on energy balances see e.g. [START_REF] Jack | Energy balances for some high gas pressure discharge lamps[END_REF][START_REF] De Groot | The High-Pressures Sodium Lamp[END_REF]. Figure 1: An example of a commonly used representation of an energy balance of a light source [START_REF] Schafer | Energy Efficient Light Sources[END_REF]. The power at the outer bulb is lost through conduction and convection in the surrounding gas, as well as thermal emission. An energy balance of a lamp burner operated in an evacuated environment and without the outer bulb looks slightly different. The UV radiation is no longer absorbed, and convection and conduction losses through the surrounding gas no longer appear.

For an HID lamp under normal operating conditions, the output energy consists of plasma radiation due to atomic line radiation, molecular radiation and continuum emission, as well as heat losses due to conduction, convection and thermal radiation. Since our measurements are performed on a lamp burner without outer bulb and operated in a vacuum system, for our situation the energy balance looks slightly different. The UV radiation is no longer absorbed, and convection and conduction losses through the surrounding gas no longer appear.

The outer bulb of the lamp absorbs part of the radiation. Therefore, it would obscure the information from the lamp burner. Particularly for the infrared this is an issue, as glass has a low transmission beyond 2.5 to 3.5 µm depending on type and thickness. For reliable infrared measurements therefore the outer bulb has to be removed. To still be able to operate the lamp for an extended time without oxidizing its metallic components and to create a representative temperature distribution of the burner, it has to be placed in vacuum. In our setup the integrating sphere is situated inside a vacuum system. The pressure in the vacuum system during the measurements is below 5•10 -5 mbar.

Operating the lamp in a vacuum system has advantages and disadvantages. One advantage is that this operation excludes heat losses through convection and conduction through surrounding gas. Therefore the only loss-process besides radiation is conduction through the connecting wires. This reduces uncertainties in the final measurements. Another advantage of operating the lamp inside a vacuum system is that this reduces water vapour and CO 2 absorptions in the infrared spectrum. A disadvantage of this technique is that after removing the outer bulb, the lamp burner operates under different conditions, and this may affect lamp performance. Another disadvantage is that due to the time required to evacuate the system it takes a few hours before lamp measurements can be performed.

The infrared integrating sphere setup

For the experiments an integrating sphere has been designed that is dedicated to the infrared, but also suitable for the visible range. The inside of the sphere was pearl blasted and subsequently coated with aluminium to obtain a diffuse reflection in a broad spectral range. For infrared measurements gold would provide a higher reflectivity, but an aluminium coating was chosen so that measurements in the visible part of the spectrum can be performed as well. A schematic drawing of the setup can be found in figure 2. The integrating sphere setup used for this experiment has an approximately spherical wall with an inner diameter of 28 cm. The wall is made of 4 mm thick copper to ensure good heat conduction. Using two parabolic mirrors, the light is sent into a Bruker Vertex80 vacuum system Fourier transform spectrometer (FTIR). A baffle is placed between the exit hole and the lamp to prevent lamp light from directly entering the spectrometer. Between the integrating sphere and the FTIR the light passes a short path outside vacuum. This path is purged with nitrogen to reduce CO 2 and water absorptions. The ratio of the spectral radiant flux leaving the integrating sphere to the spectral radiant flux of the lamp is described by (cf. equation ( 10) in [START_REF] Bergman | The Art and Science of Lamp Photometry[END_REF])

sphere hole lamp sphere A A ) ( 1 ) ( λ ρ λ ρ λ λ - = Φ Φ . ( 1
)
In this equation F l sphere is the spectral radiant flux leaving the sphere, while F l lamp represents the spectral radiant flux of the lamp. The reflectivity of the inside surface of the integrating sphere is r(l). The ratio r(l)/(1-r(l)) takes the amplification of flux due to inter-reflections of the sphere into account, A hole /A sphere is the ratio between the area of the hole and the total surface area of the integrating sphere. For this integrating sphere, the radius is about 14 cm, and the radius of the hole is about 1 cm. The reflectivity of the coating is estimated to be around 0.75. For smooth aluminium the reflectivity is above 0.95, however an oxide layer and the rough surface reduce this value. The flux leaving the integrating sphere is about 0.4% of the total lamp flux.

The integrating sphere described in [START_REF] Bergman | The Art and Science of Lamp Photometry[END_REF] has the detector directly attached to the outside of the integrating sphere. In our design projecting optics, consisting of two parabolic mirrors with an aluminium surface, is used to send the radiation from the sphere into the spectrometer. Therefore, not all of the about 0.4% of the lamp flux is projected into the spectrometer. Since also effects of the lamp holder and the baffle have to be taken into account, this integrating sphere system is to complicated to calculate the calibration function purely theoretically. Therefore a calibration using a calibration source is required. This calibration will then also include contributions from the reflectivity of the projecting optics, the transmission of the windows in the optical path and the sensitivity of the detector. The calibration procedure is discussed in chapter 3.

Comparing integrating sphere measurements to integrated side-on measurements

With the same optics one could also measure the lamp directly instead of measuring the integrated signal. For the integrating sphere described here, the ratio of the measured signal from the integrating sphere and the measured signal from a side-on setup would be about 1%. To obtain the same signal-tonoise ratio for an integrating sphere measurement as for a side-on measurement, either a much lower resolution should be selected, or a much longer integration time.

On the other hand, to accurately calculate the total emitted power of a lamp using side-on measurements, many measurements at different positions must be made and then integrated over a sphere. This requires complex rotating parts in our vacuum system. For our measurements, the integrating sphere technique is preferred.

Temperature effects

One of the problems encountered when measuring infrared radiation with wavelengths above 3 µm is that all objects around room temperature radiate in these wavelengths as well. This will give a background signal which has to be corrected for. The intensity of a black body follows from Planck's law

1 ) exp( 1 2 ) , ( 5 2 - = kT hc hc T I λ λ λ , ( 2
)
where h is Planck's constant, c the speed of light, k Boltzman's constant, l the wavelength and T the temperature. To obtain the spectral radiant flux, this equation has to be integrated over the relevant solid angle and the area of the source. For non-black bodies the radiation is found by multiplying Planck's law with the spectral emissivity e(l), which ranges from 0 to 1.

In our case, especially the background radiation from the integrating sphere itself contributes significantly to the measured signal. This is because the radiating surface of the integrating sphere is much larger than that of the lamp. Furthermore the sphere will heat up by absorption of lamp radiation, so this background radiation will not be constant. The part of the setup which is most affected by temperature effects is the baffle. Not only is the baffle the only part that is in a direct line of sight of the spectrometer, the baffle is also closer to the lamp than the rest of the sphere, and on top of that relatively loosely coupled to the rest of the sphere. Therefore, it heats up more easily than the rest of the sphere. Due to the resulting temperature difference with the rest of the sphere it also cools down much faster. To minimise temperature differences during sample-and background measurement, a temperature regulation system that runs cooling water through the baffle is operated. Heating of the surface of the baffle cannot completely be prevented, therefore a thermocouple is installed on the baffle to monitor its temperature.

Using two additional thermocouples, we measure the temperature of the integrating sphere during the lamp operation. We try to perform the background measurement at the same temperatures of the integrating sphere as during the sample measurement. The non constant temperature puts an upper limit on the allowable integration time. To improve the signal to noise ratio, the resolution of the measurements was reduced. When comparing several background corrected measurements, we find differences in their infrared signal. The differences change direction and size when comparing different measurements, but within one measurement it is a systematic error with the following profile: the error starts increasing around 3 µm and goes up to maximum 10% at 10 µm, wich is the end of our measurement range. Below 3 µm it is negligible.

Spectral ranges

The Fourier transform spectrometer can be equipped with a KBr or a CaF 2 beam splitter. For detectors we can use a room temperature DLaTGS (deuterated L-alanine doped triglycene sulphate) detector, a liquid nitrogen cooled MCT (mercury cadmium telluride) detector and a Si (Silicon) detector. For the experiments described in this paper, different combinations of detector and beamsplitter are used. The CaF 2 beamsplitter and the Si detector for are used in the interval from 780 nm to 1.1 µm; the CaF 2 beamsplitter and the MCT detector for the bandwidth from 1.1 µm to 3.3 µm and the combination of the KBr beamsplitter and the DLaTGS detector are used in the range from 3.3 µm to 10 µm. On the ports separating the integrating sphere from the Fourier transform spectrometer KBr windows are used. By combining the various beam splitters and detectors, this system has the possibility to cover a spectral range from 360 nm up to 25 µm. Due to the increasing uncertainty with increasing wavelength, we have limited our infrared measurement range to 10 µm.

Discussion of the measurement errors

Due to the non-constant temperature of the sphere and the baffle (see also section 2.4) a measurement error arises. This error starts contributing above 3 µm and yields up to 10% at 10 µm, both for the calibration and the lamp measurement, resulting in a total of 20% at 10 µm. For most of the spectral range, a random error of about 1% can be assumed. However, below about 1 µm the signal of the calibration source has decreased such that the random measurement error on the calibration function starts increasing. At 780 nm the measurement error has increased to about 20%. For the lamp measurement, a random error on the calibration function becomes a systematic error. Therefore, this contribution is mentioned separately. Over the total measurement range, a systematic error from the calculated calibration signal (4%) is added (see section 3.2),The total relative error and which sources contribute to it is shown in figure 3 for five different wavelengths.

Calibration of the integrating sphere

The key element in our calibration procedure is a resistive heated platinum strip placed inside the integrating sphere. Important properties of a calibration source are stability and reproducibility. The corrosion resistance of noble metals prevents changes in the emissivity due to oxidation. Compared to other noble metals, platinum has a good combination of a relatively high electrical resistivity with a relatively low thermal conductivity which aids in heating the strip with electric current. Our measurements show that the emission of the platinum strip is very stable; the emission changes with less than about 0.05% per hour. When the platinum strip is operated inside the integrating sphere, the measured signal S Pt (l,T Pt ) as a function of wavelength l and depending on the temperature (profile) T Pt can be written as:

), ( ) ( ) ), , ( ( ) ( ) ), , ( ( ) ( ) , ( 3 2 1
λ λ λ ε λ λ ε λ λ λ λ R k T T k T T k T S sphere sphere sphere Pt Pt Pt Pt Pt + Φ + Φ = (3)
where F is the spectral radiant flux, which depends on the emissivity e(l,T) and the temperature (profile) T. The transfer functions k x (l) take into account effects such as the the reflectance of the integrating sphere itself (k 1 (l) and k 2 (l) only), transmission of the windows, the sensitivity of the detector etc. The goal of the calibration procedure is to calculate the transfer function k 1 (l). The residual signal R(l) is the signal from radiating sources picked up along the optical path after passing the integrating sphere.

To correct for background radiation, the result of a measurement with a cold platinum strip (T Pt = T env ) is subtracted from that of the hot platinum strip. To avoid phase correction problems in the FTspectrometer data handling, the background signal is corrected for by subtracting interferograms rather than spectra [START_REF] Kember | Fourier-transform i.r. emission studies of weakly emitting overlayers on metal surfaces[END_REF][START_REF] Chase | The Sensitivity and Limitations of Condensed Phase Infrared Emission Spectroscopy[END_REF]. Academically speaking, the background measurement should be performed after removing the platinum strip from the sphere. However, the emission of the cold platinum strip at 10 µm is rougly 1.5% of the emission of the hot strip. Hence we have a systematic error of roughly 1.5%. This is much smaller than the error from the non-constant integrating sphere temperature. We measure the background radiation during cooling down of the integrating sphere. We do not have time to remove the strip from the sphere since during the removal the sphere continues to cool down. This will cause a temperature deviation of several degrees, which would result in an error in the order of 100% of the signal. We choose the smallest error, so the background measurement is done with the cold strip in the sphere. After correcting for the background signal, the transfer function k 1 (l) can be determined. It equals:

, ) ), , ( (
) , ( ) , ( ) ), , ( ( ) ), , ( (
) , ( ) , ( ) ( 1 1 1 1 1 T T T S T S T T T T T S T S k Pt Pt env Pt Pt env Pt Pt Pt Pt env Pt Pt λ ε λ λ λ ε λ ε λ λ λ λ λ λ Φ - ≈ Φ - Φ - = (4)
In this equation S Pt (l,T 1 ) and S Pt (l,T env ) are the measured signals with the hot platinum strip and the platinum strip at environmental temperature, respectively. The spectral radiant flux (F l Pt ) of the platinum strip is calculated using the model described below.

Model to calculate the temperature profile and the spectral radiant flux

The model used to determine the temperature profile and the spectral radiant flux of the strip consists of two parts. The first part is a calculation of the temperature profile; the second part calculates the spectral radiant flux resulting from this temperature profile.

Temperature profile calculation.

For the temperature profile calculations, the finite control volume method is used. The main variable of interest is the temperature and 250 control volumes are used. A one dimensional temperature profile is assumed. This is justified because the length to width ratio of the strip is about 20:1 and the width to thickness ratio is about 200:1. A direct current flowing through the strip is imposed and an initial temperature profile is assumed. As is customary for the finite volume method, for the processes dependent on the main variable (temperature), a constant value (temperature) equal to the value (temperature) of the nodal point is assumed for the complete control volume. For processes dependent on the gradient of the main value (temperature), a linear interpolation is used to determine the (temperature) gradient between the nodal points. If for processes determining the interaction between two control volumes, the value of the main variable at the control volume boundary is required, this value is also determined by linear interpolation of the values at the nodal points. A schematic drawing of the power input and power loss of each control volume is given in figure 4.

For each control volume the power balance between Ohmic heating due to dissipation in the control volume, heat conduction both into and out of the control volume and radiative losses is solved. The ohmic heating, P ohm is calculated using Joule's first law, with the resistivity from [START_REF] Kaye | Tables of physical and chemical constants[END_REF]. The conducted heat P cond is calculated using Fourier's law of thermal conduction, with the thermal conductivity from [START_REF] Touloukian | Thermophysical properties of matter part 1: thermal conductivity: metallic elements and alloys[END_REF]. The radiant flux P rad is calculated using Stefan-Boltzmann's law, with the emissivity from [START_REF] Touloukian | Thermophysical properties of matter part 7: Thermal radiative properties : metallic elements and alloys[END_REF].

The equations for all the control volumes of the strip are coupled, because the heat conduction between two neighbouring cells is equal in magnitude but of opposite sign.

The excess power (or power shortage) in each iteration is used to heat up (cool down) the control volume during a time interval dt. To determine the length of dt, a Von Neumann stability analysis for heat conduction was performed. This will give us only an estimate value, since the Ohmic heating and radiation terms were not taken into account during this stability analysis. Therefore, an additional correction factor is added. In principle this correction factor should depend on the size of the control volume. However, in our model a factor of 0.5 works well for all settings. The equilibrium profile is assumed to have been reached when the total absolute power imbalance of all cells added together is less than 1•10-5 times the total dissipated electric power.

To validate the temperature profile, the calculated Ohmic dissipation of the strip is compared to the measured input power for four different currents (7 to 10 A). The calculated Ohmic dissipation depends on the resistance of the strip, which in turn depends on the value for the thickness of the strip in the model. We have calculated the Ohmic dissipation for different thicknesses. The thickness which resulted in the lowest maximum deviation from the measured input power at all four currents is 28.14 µm. For this thickness the calculated Ohmic dissipation and the measured input power agree within 0.6% for all currents. This agrees very well with the measured thickness of the strip which is (28±1) µm.

Spectral radiant flux calculation

The second part of the model calculates the spectral radiant flux of the platinum strip from the calculated temperature profile. For each control volume of the strip the spectral radiant flux is calculated using the following equation

) , ( ) , ( ) ( T I T x xp λ λ ε π λ ∆ = Φ . (5)
In this equation ∆x is the length of each control volume and p(x) its perimeter. The spectral emissivity e(l,T) is taken from Touloukian [START_REF] Touloukian | Thermophysical properties of matter part 7: Thermal radiative properties : metallic elements and alloys[END_REF], which contains multiple datasets. We have chosen the dataset from Rolling et al. [START_REF] Rolling | Investigation of the effect of surface condition on the radiant properties of metals[END_REF] as this is the dataset with conditions best applicable to our situation. The factor π comes from the integration over the solid angle, assuming the radiation from the strip has a Lambertian profile, and I(l,T) is the black body radiation curve according to Planck's law [START_REF] Elenbaas | Light Sources[END_REF].

As verification, the calculated spectral radiant flux using equation ( 5) is integrated over the complete spectrum and compared to the radiant flux resulting from Stefan-Boltzmann's law. Both approaches should give the same results, if the correct values for the (spectral) emissivity are used. The values agree within 5% between 1000 K and 1400 K, the temperature region that causes most of the platinum strip radiation. For lower temperatures the difference goes up to 20%. For the high temperatures this difference can be caused due to the radiation not having a Lambertian profile. For the lower temperatures error is more probably a result of discrepancies between the literature values. At each temperature the spectral emissivity has been multiplied by a constant correction factor to correct for the difference. This corrected spectral emissivity is then used in equation ( 5) to calculate the spectral radiant flux of each control volume. To obtain the spectral radiant flux of the calibration source as a whole, equation ( 5) is summed over all control volumes. Temperature profiles and the radiant flux profiles are shown in figure 5 for three different currents.

Verifying the calibration function

The total emitted power depends on the current flowing through the strip. To verify the calibration, the transfer function k 1 (l), equation ( 4), is calculated for three different currents, and two measurements are performed for each current. The transfer function is a property of the system, and therefore does not depend on the current. The two measurements for each current yield results for k 1 (l) which are within 1%. When comparing two different currents, k 1 (l) varies over the wavelength axis and k 1 (l) deviates maximum 4% between one current and the other. This can have many different reasons: errors in the calculated temperature profile, error in the (literature) value of the resistivity of platinum (at high temperatures), error in the emissivity of platinum etc. However, at the moment we accept this 4% error.

Lamp Measurement

As a test case, infrared integrating sphere measurements have been performed on a CDM-T 70W/830 HID lamp. For the lamp a measurement procedure similar to the calibration measurements is performed. The measured signal with the lamp switched on inside the integrating sphere is:

) ( ) ( ) ( ) ( ) ( 3 2 1 λ λ λ λ λ λ λ R k k k S sphere lamp lamp + Φ + Φ = . ( 6
)
In this equation F l lamp is the spectral radiant flux of the lamp. For the rest, this equation is the same as equation (3). To correct for the background signal, the signal with the lamp switched off is subtracted.

Again this background correction is performed using the interferograms. The spectral radiant flux of the lamp is:

off lamp off lamp lamp lamp k S S λ λ λ λ λ Φ - - = Φ ) ( ) ( ) ( 1 . (7)
As discussed for the platinum strip calibration measurements, the background measurement is taken with the source switched off, but still inside the sphere, and subsequently neglecting the term F l lamp off .

Since k 1 (l) has been determined previously, no unknowns are left in [START_REF] Smith | Infrared losses from a Na/Sc metal-halide high intensity discharge arc lamp[END_REF] and the spectral radiant flux emitted by the lamp can be calculated. For all the three combinations of detector and beam-splitter the measurements and calculations are performed. The resulting combined spectrum is shown in figure 6.

Figure 6: The absolutely calibrated infrared integrating sphere spectrum of a CDM-T 70W/830 lamp. The bulge on the right is the thermal radiation from the burner. Almost all of the strong lines in the spectrum can be attributed to Na emission Lines.

Measurements for the visible radiation are performed at Philips Lighting Optical Calibration Laboratory (OCM) in Eindhoven [START_REF] Van Der Veen | Philips Lighting Optical Calibration Laboratory[END_REF]. The integrating sphere measurement method has been used. Their integrating sphere was calibrated with a calibration source traceable to the National Physics Laboratory (NPL) of the UK. The power in the visible part of the spectrum of this lamp is found to be 23.2±0.7 W. This value has to be adapted to our situation before we can use it in the energy balance. Firstly, this spectral power is determined for a lamp power of 71.5 W. During our infrared measurements, the lamp power was 72.3 W. If we assume that the visible power scales linearly with total power this means the visible power during our measurement was 23.8±0.7 W. Secondly, the Philips measurements were performed with a lamp inside its outer bulb. The transmission of the outer bulb is not equal to one. The visible radiation leaving the lamp burner therefore has to be scaled with a factor 1/t(l). Wharmby [START_REF] Wharmby | Energy balance of high-pressure sodium discharges under controlled vapour conditions[END_REF] has determined a value of 0.95 for the transmission of the outer bulb. Since our lamps are not the same, in our case the value could be a few percent different, but for this test case we will use 0.95 as an estimate.

The energy balance of a CDM-T 70W/830 HID lamp

The measured total power in the infrared spectrum shown in figure 6 is 35.3±3.0 W. The energy in the wavelength range above 10 µm is determined by assuming the spectral radiant flux is following the black body radiation curve, integrated over the solid angle, for a constant emissivity, and for a temperature of 1300 K. To correct for the radiating surface and the emissivity, this black body curve is scaled to connect to the measured infrared spectrum at 10 µm. With this extension, an additional 2.0±0.5 W is found in the far infrared. The power in the visible part of the spectrum of this lamp is found to be 25.0±0.7 W. About 4% of the lamp power can be attributed to UV radiation [START_REF] Van Der Veen | Philips Lighting Optical Calibration Laboratory[END_REF]. For this lamp that means 2.9 ±0.7 W. Also power loss due to conduction through the connecting wires has to be taken into account. When assuming that for the lamp burner this is comparable to the conduction loss of the platinum strip, which area and temperature are comparable to that of the burner, the conduction loss is estimated to be 3.0W±1 W. The total lamp power adds to 68.2±5.9 W. The total electric power fed to the lamp is 72.3±0.1 W. The two values agree fairly well; the difference is only 4.1 W or 5.6%. The output energy balance is shown schematically in figure 7. To fill out the complete energy balance, more measurements and subsequent data processing is required, as well as lamp modelling.

Conclusions and outlook

An integrating sphere setup for infrared measurements has been constructed, which is capable of performing absolutely calibrated integrated infrared measurements. As a test case, the infrared radiation from a CDM-T 70W/830 HID lamp has been determined. The total infrared power of the lamp can be determined with an accuracy of between 5 and 10%. This over-all accuracy is determined by the accuracy with which the spectral radiant flux can be determined. It has been shown that a significant part of the error is due to temperature differences in the integrating sphere between the sample measurement and background measurement. In the near future we plan to further improve the accuracy of the calibration by implementing a better temperature control in the integrating sphere. Furthermore we plan to implement a dynamic switch-off experiment in order to make the distinction between IR plasma continuum radiation and radiation from the lamp wall.

A

  calibrated integrating sphere setup to determine the infrared spectral radiant flux of high-intensity discharge lamps a J Rijke, S Nijdam, M Haverlag, J F J Janssen, J J a M van Der Mullen, G M W Kroesen To cite this version: a J Rijke, S Nijdam, M Haverlag, J F J Janssen, J J a M van Der Mullen, et al.. A calibrated integrating sphere setup to determine the infrared spectral radiant flux of high-intensity discharge lamps. Journal of Physics D: Applied Physics, 2011, 44 (22), pp.224007. 10.1088/0022-3727/44/22/224007. hal-

Figure 2 :

 2 Figure 2: A schematic representation of the setup. The FTIR and the part of the setup containing the integrating sphere are evacuated. Between the integrating sphere and the FTIR the light passes a short path outside vacuum. This path is purged with nitrogen.

Figure 3 :

 3 Figure 3: A bar chart indicating the error for different wavelengths and their cause. The white block is the error from the calculated emission of the calibration source. The dashed bar indicates the error due to the decreasing signal of the calibration source. The bars with the horizontal stripes indicate the error from differences in the temperature of the integrating sphere between sample and background measurement. The gray bars indicate the random errors on the measurement.

Figure 4 :

 4 Figure 4: A schematic drawing of the power input and power loss of each control volume. The horizontal arrow pointing into the control volume indicates the power (P Ohm ) going into the control volume by Ohmic heating. In equilibrium, this Ohmic heat must equal the sum of the net power radiated away (P rad , indicated by the horizontal double arrows pointing out of the control volume), and the conducted heat (P cond , indicated by the arrows between the control volumes). The conducted power leaving one control volume equals the power entering the neighbouring control volume.

Figure 5 :

 5 Figure 5: The temperature profile (top) of the strip and the resulting radiant flux profile (bottom) for different currents (from top to bottom 10, 9 and 8 ampere). A smooth profile with a large plateau in the centre and steep edges is found.

Figure 7 :

 7 Figure 7: Energy balance of a CDM-T 70W/830 HID lamp. The burner is operated in vacuum and without outer bulb.

Acknowledgements

Peter Weerdesteijn, Jannet van der Veen and Jos Suijker are acknowledged for their contributions to the experimental setup, and for determining the visible and the UV contributions to the energy balance.


Enthalpies of formation, entropies & heat capacities of lanthanide monohalides Standard enthalpies of formation, entropies and heat capacities of the lanthanide

Introduction

The lanthanide monohalides, LnX<g>, are important radiating species in high pressure plasmas derived from doses containing lanthanide(III) halides. To understand the characteristics of these plasmas and to determine the role of the lanthanide monohalides it is necessary to calculate the chemical composition of the plasma from the coolspot temperature (typically 1000 to 1500 K) to the arc centre temperature (typically in the range 4000 to 6000 K). Such calculations are usually based on the assumption of local thermodynamic equilibrium, LTE. A typical chemical system used in a ceramic metal halide discharge lamp might contain a mixture of the iodides of Sodium, Thallium, Holmium and Thulium (collectively known as the 'lamp dose') together with Mercury and Argon. The coolspot temperature of the vessel containing the plasma determines the composition of the liquid and hence the elemental composition of the gas phase above the dose. These compositions in turn determine the elemental composition and hence the species concentration throughout the plasma chamber from the wall temperature to the highest temperatures found in the centre of the arc.

Detailed knowledge of the temperature variation of chemical composition within the plasma is an essential precursor to predicting the corresponding transport coefficients. The temperature variation of the transport coefficients not only govern the microscopic properties of the plasma but also have to be taken into consideration in the design and fabrication of the lamp and its components. Of the transport coefficients: viscosity controls convection within the plasma chamber and has a major influence on the power loss and hence the efficiency of the lightsource; the electrical conductivity of the plasma determines the voltage of the lamp and plays a vital role in the design of the ballasts needed to control the electrical characteristics of high intensity discharge lamps; the thermal conductivity of the plasma is another contributor to power loss and determines the wall temperature of the plasma chamber which in turn has a major influence on the failure mechanisms and life of the device. The temperature variation of the heat capacity of the plasma reflects changes in chemical composition. These changes are responsible for the concentrations of radiating and charged species.

To calculate the plasma composition, thermodynamic functions for all of the species under consideration must be available and held in the appropriate database format. None of the readily available thermochemical databases contain a complete set of self consistent values for the lanthanide monohalides. This paper outlines the determination of the molar standard enthalpies of formation, molar entropies and molar heat capacities for the series LnX<g> (Ln = Ba, La to Lu and X = F, Cl, Br, I) from dissociation energies and molecular constants.

In lamps with electrodes the use of lanthanide halides is restricted to the bromides and iodides. Electrodeless plasma lamps, however, allow chloride-containing systems to be used. Fluorides are not used in discharge lamps but they are included here to complete the LnX<g> series. Promethium, Pm, is an unstable element Confidential: not for distribution. Submitted to IOP Publishing for peer review 7 March 2011 and does not occur naturally but it is included in all theoretical considerations of lanthanide compounds. Although Barium is not a lanthanide element it is often included in studies of the lanthanide series because of its position in the Periodic Table . Preliminary accounts of this work have been given earlier [START_REF] Mucklejohn | Proc. 12th Int. Symp. Science & Technology Light Sources and 3rd Int. Conf[END_REF], [START_REF] Mucklejohn | Proc. 12th Int. Symp. Science & Technology Light Sources and 3rd Int. Conf[END_REF].

A detailed account of the behaviour and use of metal halides and other molten salts in high intensity discharge lamps is given in [START_REF] Mucklejohn | Proc. Int. George Papatheodorou Symp[END_REF].

Molecular constants

Molecular constants for LnF<g> and LnCl<g> except BaF<g>, BaCl<g>, LaF<g> and LaCl<g> were taken from reference [START_REF] Chervonnyi | [END_REF]. Molecular constants for LaF<g>, LaCl<g>, LaBr<g> and LaI<g> were taken from reference [5]. Molecular constants for BaF<g> and BaCl<g> were taken from the NIST Chemistry WebBook [6].

In the absence of a published set of self consistent values for the molecular constants for the series LnBr<g> and LnI<g>, these parameters were estimated. As the molecular constants had to be estimated, the terms for anharmonicity contributions to the molar heat capacities and entropies, i.e. the anharmonicity constant (ω e x e ), the rotational constant (B e ) and the rotational -vibrational constant (α e ), were omitted. Values of the vibrational wavenumbers (ω e ) and the equilibrium interatomic distances (R e ) for the series LnBr<g> and LnI<g> were estimated using the ratios of the corresponding values for LaCl:LaBr and LaCl:LaI from [5].

The energy levels and quantum weights selected for the LnX<g> molecules were those for the corresponding Ln(II) species [7]. A similar approach to representing the energy levels of LnX<g> molecules [START_REF] Chervonnyi | [END_REF] and LnI 3 <g> molecules [START_REF] Mucklejohn | Proc. 3rd Int Symp. High Temp. Lamp Chem. editors[END_REF] has been used previously to reflect the ionic character of the Ln-X bonds.

The molecular constants used in this work are listed in tables 1 to 4.

Heat capacities and entropies at 298.15 K

The heat capacities for the LnX<g> series were calculated from 298.15 to 6000 K using the 'StatMech' software [START_REF]StatMech is the statistical mechanics module of MTDATA Studio[END_REF] with input values for the molecular constants listed in tables 1 to 4. The coefficients for the temperature variations of the molar heat capacities (C p,m (T)) of the lanthanide monohalides, expressed as [START_REF] Mucklejohn | Proc. 12th Int. Symp. Science & Technology Light Sources and 3rd Int. Conf[END_REF] are listed in tables 5 to 8.

C p,m (T)/J.K -1 .mol -1 = A + BT + CT 2 + DT -2 (1)
where T = temperature (K)

Molar heat capacities for Ln<g> and X<g> were calculated using energy levels for Ln(I) and X(I) as listed in [7].

These calculations also provide the molar entropies (S m ) at 298.15 K for the lanthanide monohalides, these are listed in table 9.

Derivation of standard enthalpies of formation from dissociation energies

The standard enthalpies of formation for LnX<g> were derived from scheme 1. The dissociation energies for LnX<g> were taken from reference [START_REF] Kaledin | [END_REF], the standard enthalpies of formation for Ln<g> and X<g> were taken from the SGTE Substance Database [11]. where <r> is the standard state of the element at the reference temperature (298.15 K)

(2) = ( 4) + ( 5) -(

Dissociation energies in table 10 are taken to be at 0 K, the corresponding enthalpies of dissociation at 298.15 K were determined from equations ( 3), ( 7) and [START_REF] Mucklejohn | Proc. 3rd Int Symp. High Temp. Lamp Chem. editors[END_REF].

∆C p (T) = C p (Ln,g,T) + C p (X,g,T) -C p (LnX,g,T) ( 7)

∆H(T) = ∆H(298.15 K) + Τ 298.15 Κ ∆C p (T)dT (8)
Molar standard enthalpies of dissociation at 298.15 K for the lanthanide monohalides derived from this study are presented in table 11. The molar standard enthalpies of formation for the lanthanide monohalides are shown in table 12.

Discussion

While the molecular constants have a substantial effect of the calculated values of the molar heat capacities (C p,m ) and entropies (S m (T)) it is the impact on the temperature variation of the standard Gibbs free energy of formation (∆G m (T)) that is important when considering the effect of these parameters on the calculated chemical composition of a plasma. The values between 1000 and 6000 K are therefore of most interest in this application. Taking LaBr<g> as an example, the value of S m (1000 K) is 330.656 J.K -1 .mol -1 with energy levels included in the calculation and 312.307 J.K -1 .mol -1 without the energy levels. The corresponding values at 6000 K are 379.305 and 409.017 J.K -1 .mol -1 . The impact of these changes on the standard Gibbs free energy can be illustrated by ∆G m (1000 K) with energy levels included in the calculation of C p,m (T) and S m (T) is -190641 J.mol -1 whereas without the energy levels the value would be -208862 J.mol -1 . The corresponding values at 6000 K are -1936006 J.mol -1 and -2113460 J.mol -1 respectively. The inclusion of energy levels has an impact of approximately 9 per cent on the value of ∆G m (LaBr,g,T) in the range 1000 to 6000 K.

The effect of changing the value of the vibrational wavenumber (ω e ) on the calculated value for the standard Gibbs free energy of formation for LaBr<g> can be illustrated by varying the selected value (237.8 cm -1 ) by +/-10 per cent. Such changes result in differences of less than 1 per cent in the values of ∆G m (T) in the range 1000 to 6000 K.

Similar effects are seen if the value of the equilibrium interatomic distance (R e ) for LaBr<g> is changed by +/-10 per cent. These changes also result in differences of less than 1 per cent in the values of ∆G m (T) in the range 1000 to 6000 K.

Anharmonicity contributions to the molar heat capacities and entropies were included for LnF<g> and LnCl<g> but not included for LnBr<g> and LnI<g>. The anharmonicity contributions for the LnX<g> series increase with temperature and with the increasing atomic mass of the halogen. However, as described above, it is the impacts on the values of the standard Gibbs free energies of formation that are important for this application. The anharmonicity contributions to the molar heat capacities of LaF<g> and LaCl<g> at 1000 K are 0.314 J.K -1 .mol -1 and 0.399 J.K -1 .mol -1 respectively. These represent 0.007 and 0.008 of the values of the molar heat capacities. The corresponding values at 6000 K are 0.040 and 0.049. The impacts on the values of the molar entropies are lower, being 0.001 for both LaF<g> and LaCl<g> at 1000 K and 0.005 and 0.006 respectively at 6000 K. The differences between ∆G m (LaCl,g,T) calculated with and without anharmonicity contributions are less than 1 per cent throughout the temperature range 1000 to 6000 K. The equivalent differences for the LnBr<g> and LnI<g> series are estimated to be less than 2 per cent.

The molar standard enthalpies of formation at 298.15 K and the molar entropies at 298.15 K for the Barium monohalides are listed in the SGTE Substance database [11] and are shown in table 13 together with the corresponding values from reference [6] (although this source does not provide values for BaCl<g>).

Comparison of these values with the results reported in table 9 show that the molar entropies at 298.15 K are within 1.2 J.K -1 .mol -1 for all four of the Barium monohalides.

Values for the molar standard enthalpies of formation at 298.15 K, however, show substantial differences between the three sources. While the ranges of values for BaF<g> are 6527 J.mol -1 and 3320 J.mol -1 for BaCl<g>, the ranges for BaBr<g> and BaI<g> are much greater. The SGTE database lists a values of -75324 J.mol -1 for ∆ f H m (BaBr,g,298.15 K), the NIST source gives -110590 J.mol -1 compared to -63048 J.mol -1 from this work. There are also large differences in the values reported for ∆ f H m (BaI,g,298.15 K): -10237 J.mol -1 (SGTE); -42420 J.mol -1 (NIST); -29265 J.mol -1 (this work).

This spread of values exemplifies the need for a set of self consistent values for lanthanide and Barium monohalides.

Conclusions

From the molar standard enthalpies of formation, the molar entropies at 298.15 K and the temperature variation of the molar heat capacities a dataset for each of the LnX<g> series can be derived in the SGTE format [11], or equivalent, for use with integrated thermochemical databank packages.

For the calculation of chemical compositions in high pressure plasmas it is the temperature variations of the standard Gibbs free energies of formation for all of the species being considered that are important. Thus, for example, values for ∆ f G m (LnX,g,T) must be consistent with those for ∆ f G m (Ln,g,T), ∆ f G m (X,g,T), ∆ f G m (Ln + ,g,T) and ∆ f G m (LnX + ,g,T). Reference [11] Reference [6] ∆ f H m (J.mol -l ) S m (J.K -1 .mol -1 ) ∆ f H m (J.mol -1 ) S m (J.K -1 .mol

Scheme 1 . 4 ) 1 /

 141 Scheme 1. Derivation of standard enthalpy of formation from dissociation energy Ln<r> + 1/2X 2 <r> = LnX<g> ∆H = Standard enthalpy of formation of LnX<g> (2) LnX<g> = Ln<g> + X<g> ∆H = Enthalpy of dissociation of LnX<g> (3) Ln<r> = Ln<g> ∆H = Standard enthalpy of formation of Ln<g> (4)

Table 1 .

 1 Molecular constants for LnF<g> used in this work.

	Ln	Molar mass (g.mol -1 )	ω e (cm -1 ) ω e x e (cm -1 ) B e (cm -1 )	α e (cm -1 )	R e (cm)	Reference
	Ba	156.338	468.9	1.790	0.2158	1.2000E-03 2.162E-08
	La	157.908	575.2	2.133	0.2464	1.2217E-03 2.023E-08
	Ce	159.118	581.2	2.170	0.2483	1.2400E-03 2.048E-08
	Pr	159.905	586.9	2.210	0.2505	1.2600E-03 2.005E-08
	Nd	163.238	592.1	2.250	0.2521	1.2800E-03 1.996E-08
	Pm	163.998	596.7	2.280	0.2542	1.3000E-03 1.987E-08
	Sm	169.348	600.8	2.320	0.2553	1.3100E-03 1.978E-08
	Eu	170.958	493.1	1.900	0.2300	1.2100E-03 1.935E-08
	Gd	176.248	607.6	2.380	0.2582	1.3400E-03 1.963E-08
	Tb	177.922	605.4	2.400	0.2599	1.3600E-03 1.955E-08
	Dy	181.498	607.4	2.420	0.2612	1.3700E-03 1.948E-08
	Ho	183.928	613.9	2.440	0.2626	1.3800E-03 1.941E-08
	Er	186.258	615.0	2.450	0.2640	1.3900E-03 1.935E-08
	Tm	187.932	615.5	2.450	0.2654	1.4000E-03 1.928E-08
	Yb	192.038	505.5	1.900	0.2422	1.2500E-03 2.016E-08
	Lu	193.968	615.1	2.450	0.2676	1.4100E-03 1.917E-08

Table 2 .

 2 Molecular constants for LnCl<g> used in this work.

	Ln	Molar mass (g.mol -1 )	ω e (cm -1 ) ω e x e (cm -1 ) B e (cm -1 )	α e (cm -1 )	R e (cm)	Reference
	Ba	172.793	279.3				2.683E-08
	La	174.363	341.6	0.98	0.1	3.6000E-04 2.498E-08
	Ce	175.573	344.3	1.00	0.1	3.6600E-04 2.486E-08
	Pr	176.360	346.9	1.02	0.1	3.7200E-04 2.475E-08
	Nd	179.693	349.1	1.05	0.1	3.7800E-04 2.464E-08
	Pm	180.453	351.0	1.06	0.1	3.8400E-04 2.453E-08
	Sm	185.803	352.6	1.09	0.1	3.5600E-04 2.442E-08
	Eu	187.413	288.7	0.89	0.1	3.9400E-04 2.571E-08
	Gd	192.703	354.9	1.12	0.1	4.0000E-04 2.423E-08
	Tb	194.377	355.6	1.14	0.1	4.0500E-04 2.413E-08
	Dy	197.953	356.0	1.15	0.1	4.1000E-04 2.404E-08
	Ho	200.383	356.1	1.17	0.1	4.1500E-04 2.396E-08
	Er	202.713	355.9	1.18	0.1	4.2000E-04 2.388E-08
	Tm	204.387	355.3	1.18	0.1	3.8000E-04 2.379E-08
	Yb	208.493	291.1	0.92	0.1	2.3660E-04 2.488E-08
	Lu	210.423	353.4	1.19	0.1	3.6399E-04 2.366E-08

Table 3 .

 3 Molecular constants for LnBr<g> used in this work.

	Ln	Molar mass (g.mol -1 )	ω e (cm -1 )	R e (cm)
	Ba	217.244	194.4	2.840E-08
	La	218.814	237.8	2.644E-08
	Ce	220.024	239.7	2.631E-08
	Pr	220.811	241.5	2.620E-08
	Nd	224.144	243.0	2.608E-08
	Pm	224.904	244.4	2.596E-08
	Sm	230.254	245.5	2.585E-08
	Eu	231.864	201.0	2.721E-08
	Gd	237.154	247.1	2.565E-08
	Tb	238.828	247.6	2.554E-08
	Dy	242.404	247.8	2.544E-08
	Ho	244.834	247.9	2.536E-08
	Er	247.164	247.8	2.527E-08
	Tm	248.838	247.3	2.518E-08
	Yb	252.944	202.7	2.633E-08
	Lu	254.874	246.0	2.504E-08

Table 4 .

 4 Molecular constants for LnI<g> used in this work.

	Ln	Molar mass (g.mol -1 )	ω e (cm -1 )	R e (cm)
	Ba	264.244	153.2	3.082E-08
	La	265.814	187.4	2.869E-08
	Ce	267.024	188.9	2.855E-08
	Pr	267.811	190.3	2.843E-08
	Nd	271.144	191.5	2.830E-08
	Pm	271.904	192.6	2.817E-08
	Sm	277.254	193.5	2.805E-08
	Eu	278.864	158.4	2.953E-08
	Gd	284.154	194.7	2.783E-08
	Tb	285.828	195.1	2.772E-08
	Dy	289.404	195.3	2.761E-08
	Ho	291.834	195.4	2.752E-08
	Er	294.164	195.3	2.743E-08
	Tm	295.838	194.9	2.732E-08
	Yb	299.944	159.7	2.858E-08
	Lu	301.874	193.9	2.718E-08

Table 5 .

 5 Coefficients for the temperature variation of the molar heat capacities of the lanthanide monofluorides as expressed in equation (1), 298.15 to 6000 K.

	Ln	A	B	C	D	T max
	BaF	3.9487E+01 -6.9904E-03	6.4499E-06 -2.8676E+05
		9.4870E+00	2.7479E-02 -4.0859E-06	5.7968E+06
		2.9618E+01	1.9553E-02 -4.1888E-06 -1.2598E+07
		8.3099E+01 -1.2628E-02	1.0845E-06 -4.9295E+07
		4.1514E+01 -4.9070E-04	9.0780E-08	9.5070E+07
	LaF	1.2776E+01	9.4742E-02 -6.4110E-05	9.1960E+04
		7.1988E+01 -2.7690E-02	7.0616E-06 -4.0030E+06
		4.1701E+01 -1.0918E-03	3.9272E-07	8.2930E+06
		1.1920E+01	8.7033E-03 -5.1407E-07	9.1047E+07
	CeF	3.0739E+01	3.1653E-02 -9.1755E-06 -2.0780E+04
		4.3332E+01	1.9666E-02 -7.3462E-06 -2.6914E+06
		7.8992E+01 -1.3928E-02	1.4188E-06 -1.3825E+07
		4.5365E+01 -1.4267E-03	1.1034E-07	5.9576E+07
	PrF	3.3229E+01	1.8073E-02 -4.6550E-06 -3.6900E+03
		6.8879E+01 -8.8117E-03	7.6430E-07 -1.6926E+07
		5.0355E+01 -2.7802E-03	2.4115E-07	3.0077E+07
	NdF	3.6780E+01	1.6719E-02 -6.1974E-06 -2.3120E+05
		4.0243E+01	8.6624E-03 -1.5880E-06 -1.8882E+05
		7.3773E+01 -7.1694E-03	4.8510E-07 -4.2561E+07
	PmF	3.5320E+01	4.7616E-03 -9.8968E-07 -2.4467E+05
		3.9815E+01	1.3861E-03 -1.8855E-07 -2.8738E+06
		4.5296E+01 -9.9170E-04	8.3320E-08 -8.6346E+06
	SmF	3.6651E+01	2.8354E-02 -1.5563E-05 -1.2250E+05
		6.2314E+01 -1.2368E-02	2.4113E-06 -3.0168E+06
		4.8603E+01 -9.7310E-04	5.8410E-08 -3.4653E+06
	EuF	3.0190E+01	2.0213E-02 -9.9722E-06 -6.0190E+04
		4.8668E+01 -8.2348E-03	2.3319E-06 -2.3624E+06
		1.4176E+01	1.2073E-02 -9.6631E-07	2.4644E+07
		5.7160E+01	3.7570E-04 -7.4010E-08 -1.4672E+08
	GdF	4.3845E+01 -2.8997E-03	3.7786E-06 -2.7556E+05
		4.2442E+01	4.9399E-03 -1.1271E-06 -1.8069E+06
		5.7553E+01 -3.7802E-03	2.7788E-07 -1.5200E+07
	TbF	3.7386E+01	4.7645E-03	3.2000E-06 -3.8448E+05
		2.4676E+01	2.9931E-02 -1.0059E-05	2.9537E+05
		6.6845E+01 -1.1992E-02	1.4698E-06 -1.1455E+07
		3.8702E+01 -5.9730E-04	1.6295E-07	4.1523E+07
	DyF	4.1725E+01 -7.3400E-04 -1.0704E-06 -5.2049E+05
		3.6867E+01	1.6932E-03	6.9605E-07	6.0312E+05
		3.0644E+01	7.8199E-03 -8.2667E-07	8.4000E+05
		6.2021E+01 -2.6069E-03	1.4505E-07 -8.3279E+07

Table 6 .

 6 Coefficients for the temperature variation of the molar heat capacities of the lanthanide monochlorides as expressed in equation (1), 298.15 to 6000 K.

	Ln	A	B	C	D	T max
	BaCl	4.0921E+01 -1.0289E-02	8.0811E-06 -2.0528E+05
		-7.8160E+00	4.4604E-02 -9.0096E-06	1.0728E+07
		1.0006E+02 -1.9696E-02	1.8065E-06 -8.0256E+07
		9.1621E+03 -2.8633E+00	2.5260E-04 -2.7299E+10
		8.9394E+01 -1.2733E-02	9.0864E-07 -1.3464E+08

Table 7 .

 7 Coefficients for the temperature variation of the molar heat capacities of the lanthanide monobromides as expressed in equation (1), 298.15 to 6000 K.

	Ln	A	B	C	D	T max
	BaBr	4.1318E+01 -1.1200E-02	8.6494E-06 -1.7040E+05
		-1.0092E+01	4.6353E-02 -9.3768E-06	1.1824E+07
		9.5727E+01 -1.7795E-02	1.5770E-06 -7.4012E+07
		4.4865E+01 -1.7192E-03	1.4561E-07	7.8415E+07

Table 8 .

 8 Coefficients for the temperature variation of the molar heat capacities of the lanthanide monoiodides as expressed in equation (1), 298.15 to 6000 K.

	Ln	A	B	C	D	T max
	BaI	4.1415E+01 -1.1441E-02	8.8099E-06 -1.5478E+05
		-1.0325E+01	4.6514E-02 -9.4079E-06	1.1980E+07
		9.5727E+01 -1.7795E-02	1.5770E-06 -7.3991E+07
		4.4849E+01 -1.7152E-03	1.4534E-07	7.8515E+07

Table 9 .

 9 Molar entropies a (S m ) for LnX<g> at 298.15 K.

	Ln	LnF (J.K -1 .mol -1 ) LnCl (J.K -1 .mol -1 ) LnBr (J.K -1 .mol -1 )	LnI (J.K -1 .mol -1 )
	Ba	246.143	257.805	269.902	277.791
	La	252.320	264.332	275.522	283.351
	Ce	257.090	268.003	282.493	290.316
	Pr	259.955	271.799	283.119	290.938
	Nd	258.612	270.484	281.745	289.551
	Pm	278.456	290.411	301.573	309.374
	Sm	252.742	264.677	275.819	283.600
	Eu	258.973	270.451	282.569	290.404
	Gd	261.455	273.448	284.495	292.255
	Tb	263.034	275.043	286.025	293.784
	Dy	264.051	276.088	287.020	294.772
	Ho	262.477	274.590	285.488	293.222
	Er	263.924	276.082	286.927	294.663
	Tm	261.595	273.802	284.599	292.324
	Yb	250.705	262.572	274.246	282.026
	Lu	260.103	252.204	283.124	290.835

a Values are expressed to 3 decimal place by convention and do not represent accuracy

Table 12 .

 12 Molar standard enthalpies of formation (∆ f H m ) for LnX<g> at 298.15 K.LnLnF (J.mol -1 ) LnCl (J.mol -1 ) LnBr (J.mol -1 ) LnI (J.mol -1 ) Ba

		-315629	-132970	-63048	-29265
	La	-159888	22772	89135
	Ce	-125185	78420	153748
	Pr	-146769	51461	121400
	Nd	-142230	27289	97228
	Pm	-222781	-36529	33428	66807
	Sm	-280199	-95152	-16810	16569
	Eu	-287401	-110956	-40778	-7424
	Gd	-116718	65939	135908
	Tb	-181817	33165	111479
	Dy	-166421	11476	81417
	Ho	-138107	9890	88204
	Er	-179394	-15842	62473
	Tm	-196975	-26242	43698	77078
	Yb	-293188	-103292	-33386	-31
	Lu	-16193	165899	236447

Table 13 .

 13 Molar standard enthalpies of formation (∆ f H m ) and molar entropies (S m ) for BaX<g> at 298.15 K from the SGTE Substance database[11] and the NIST Chemistry WebBook [6].

Table10. Dissociation energies from reference [START_REF] Kaledin | [END_REF].

Ln

LnF (cm -1 ) LnCl (cm -1 ) LnBr (cm -1 ) LnI (cm


M Willander

O Nur

S Zaman

A Zainelabdin

N Bano

I Hussain

Zinc Oxide Nanorods/Polymer Hybrid Heterojunctions for White Light Emitting Diodes

Zinc oxide (ZnO) with its deep level defect emission covering the whole visible spectrum holds promise for the development of intrinsic white lighting sources with no need of using phosphors for light conversion. ZnO nanorods grown on flexible plastic as substrate using a low temperature approach (down to 50 o C) were combined with different organic semiconductors to form hybrid junction. White electroluminescence (EL) was observed from these hybrid junctions. The configuration used for the hybrid white light emitting diodes (LEDs) consists of two-layers of polymers on the flexible plastic with ZnO nanorods (NRs) on the top. The inorganic/organic hybrid heterojunction has been fabricated by spin coating the p-type polymer Poly (3,4ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) for hole injection with an ionization potential of 5.1 eV and Poly(9, 9-dioctylfluorene) (PFO) is used as blue emitting material with a bandgap of 3.3 eV. ZnO nanorods (NRs) are grown on top of the organic layers. Two other configurations were also fabricated; these are using a single MEH PPV (red emitting polymer) instead of the PFO and the third configuration was obtained from a blend of the PFO and the MEH PPV. The white LEDs were characterized by scanning electron microscope (SEM), x-ray diffraction (XRD), currentvoltage (I-V) characteristics, room temperature photoluminescence (PL) and electroluminescence (EL). The EL spectrum reveals a broad emission band covering the range from 420 to 800 nm, and the emissions causing this white luminescence were identified.

Introduction

Zinc oxide (ZnO) has a direct wide bandgap of 3.37 eV and a relatively large exciton biding energy of 60 meV. Although ZnO is well known to researchers since the 30s, it has now become very attractive for researchers world wide [1,2]. The sudden huge increase of the interest in this material is due to many reasons. The self organized growth property of ZnO, enables the growth of its nanostructures on any substrate, being amorphous or crystalline in nature and hence pn heterojunctions of device quality can be achieved using external p-type materials [3]. This is in addition to the possibility of low temperature aqueous chemical growth (< 100 o C) of these nanostructures [4] and the interesting effect of lowering the growth temperature using this regime of growth.

Moreover, ZnO is bio-safe and biocompatible [5]. In addition, ZnO possesses a large number of deep level defects emissions covering the whole visible region, as ZnO emits violet, blue, green, yellow, orange and red lines [6]. These advantageous have led to the sudden increase of interest on ZnO as a material of potential for many applications in recent years. Due to the large bandgap, UV lasers and white light emitting diodes (LEDs) and detectors are possible to achieve using heterojunctions of ZnO nanostructures with other materials. The possibility to obtain different emission colors from ZnO will be of potential for the development of white light emitting diodes with no need for light conversion material like in e.g. GaN LEDs. This is in addition to the possibility to improve existing LEDs, e.g. combing ZnO and GaN will lead to avoid the use of phosphors for light conversion.

Here we demonstrate and characterize white LEDs based on the combination of ZnO nanorods and organic semiconducting polymers. Although pure organic LEDs (OLEDs) emitting white light have been demonstrated, the present LED offers some advantageous over pure OLEDs. Among them is the fact that we avoid using n-type polymers which suffer from ´instability. In addition the use of ZnO nanorods will enable the fabrication of large area LEDs with single contact. This is due to the many discrete pn junction formed by single ZnO nanorods. This implies that using a single contact to the ZnO nanorods through a top conducting transparent layer a large area white LED can be achieved. This would not be possible to achieve using pure OLEDs.

Recently ZnO have been demonstrated as a material of potential for many applications e.g. photonics, bio-and chemical sensors etc. [1,5]. The combination of ZnO in the nanostructure form with organic semiconducting or conducting layers for photonic applications has been reported since 2004 [7]. In this paper we present our recent findings on the growth of ZnO nanorods NRs using the chemical growth approach.

We discuss and analyze the effect of lowering the temperature on the structural and optical properties of the NRs. White light electroluminescence from two different heterojunctions composed of ZnO NRs with organic polymers will be presented and discussed.

Experimental

Zinc oxide NRs presented here were grown by the aqueous chemical growth at low temperatures (96 o C and down to 50 o C). The procedure is a two step process. The first step is the use of a ZnO nano-particle seed layer to act as the nucleation site.

Although the growth of ZnO nanostructures benefit from the self organization growth property of this material, the use of the seed layer is essential for obtaining uniform and well aligned NRs. Hence care must be taken in preparing the seed layer solution and during the coating of the substrate with this solution. Usually the seed layer deposition is followed by post deposition temperature annealing in the temperature range of (250-350 o C). We have not employed any temperature annealing of the seed layer after coating, but concentrated our effort to prepare homogenous seed layer solution since it has a major effect on the uniformity of the growth of the nanostructures. In the present devices the seed layer was deposited 3-5 times to insure uniform distribution of the ZnO nanoparticles over the whole substrate area. All the chemicals used were commercially purchased from Sigma-Aldrich and were used with no further purification. The aqueous solutions were prepared using clean room deionized water. The zinc oxide nano-particle seed layer solution was prepared using mixing Zinc Acetate Dehydrate (C4H6O4Zn.2(H2O) in methanol aiming at 0.01M concentration. Then this solution was heated up to 60 oC. A drop-wise solution of KOH in methanol (0.03M) was added under vigorous stirring and the whole solution was kept at 60 oC for 2 hours. Such solution is expected to contain ZnO nanoparticles with particle size in the range 3-5 nm [8]. The nanoparticles solution was then spun coated on the cleaned substrates many times at spin speed of 2000 rpm for 40s. Another aqueous solution was prepared by dissolving Zinc Nitrate Hexahydrate (Zn(NO3)2.6H2O) in 100mL deionized water to achieve (0.15M) targeted concentration, and Hexamethylenetetramine (HMT, C6H12N4) in 100 mL DIwater to obtain (0.1M) solution. The two solutions were transferred to a 250 mL beaker under stirring for 5 minutes, pre-coated substrates with then immediately submerged horizontally inside the beaker containing the precursor solution. Then the reaction containers were directly inserted in a regular laboratory oven kept at a fixed temperature.

We have used temperatures ranging between 96 o C and 50 o C for several hours. After the chosen reaction time elapsed, the substrates were thoroughly soaked in deionized water under ultrasonic agitation to remove any residual salts and then were dried at room temperature. Different substrates were used as mentioned above. Here we choose to show results obtained using flexible plastic (coated with PEDOT:PSS as examples of the use amorphous substrates, respectively. After growth different techniques were used to characterize the structural and optical properties of the grown ZnO NRs. This was achieved by using scanning electron microscopy (SEM), and x-ray diffraction (XRD). In addition, the electro-optical properties were also investigated using photoluminescence (PL) and electroluminescence (EL). Finally the color quality was extracted to obtain the color rendering index of the emitted light from ZnO NR/p-polymer hybrid junction LEDs fabricated using different polymers to investigate the effect of the polymer emission of the LED performance. The SEM morphology analysis was performed using JEOL JSM-6301F equipment. While the x-ray diffraction (XRD) was measured with Cu K line at wavelength 1.54Å. Room temperature PL measurements were performed using a Coherent MBD266 laser operating at = 266 nm. The EL behavior of the fabricated devices was examined with Keithley 2400 source meter, while the EL spectra were assembled via Andor Shamrock 303iB spectrometer supported with Andor-Newton DU-790N CCD. debate for many years. Moreover, to reduce the native defects and obtain better optical properties of ZnO NRs grown at 95 o C, usually the NRs are annealed [9]. Here we will discuss and show a route for obtaining ZnO nano-crystal with good optical quality.

Results and discussion

Nevertheless, the discussion below will reveal that lowering the temperature more than 95 o C will in fact not degrade the crystalline quality of the grown ZnO NRs. Another crystal quality ZnO NRs with less radiative point defects accompanied by morphological changes will be obtained when the growth temperature is reduced. As mentioned above the samples presented here were grown at different temperatures including 95 o C, 75 o C and 50 o C. We have observed that as the growth temperature is increased the length of the ZnO NRs is increased and in addition the grown NRs are becoming thinner in diameter.

The scenario proposed involves two cases. For the highest growth temperature i.e. (>90 o C) the seed layer particle size of the critical radius of nucleation decreases compared to the cases of growth at lower temperatures, while the nucleation rate increases as a consequence of increasing the zinc solubility as the temperature increases. In other words, at higher temperatures there is a competition between the nucleation on the substrate surface and the axial growth of the NRs. This implies that for the same growth parameters, the same solution concentration, and growth duration, this competition will lead to shorter and thicker NRs as the temperature is increased. At the same time the density is maximized when the temperature is increased. On the other hand as the temperature of growth is decreased i.e. growth at 50 o C, the critical radius of nucleation remains constant over the growth duration and as a consequences longer ZnO NRs are obtained. This suggested scenario is in fact consistent with our observation on the morphology of the ZnO NRs grown at different temperatures. It is important to mention that regardless of the substrate surface used and the lattice mismatch to ZnO crystal.

These results indicate that the controllability over ZnO NRs can be achieved by optimizing the growth parameters independent of the substrate used. In fact this is an important finding as it allows the growth of ZnO NRs on almost any substrate. This also confirms the self organized growth property of this material. Another important consequence of lowering the growth temperature is the concentration of the deep level native defects. As mentioned above, it was believed that NRs grown at lower temperature possesses a large amount of deep level defects compared to samples grown at higher temperatures. The ZnO NRs were grown at 95 o C as described above. Figure 3 shows the room temperature PL spectra of the PFO (without ZnO NRs) and for the ZnO NRs/PFO/PEDOT:PSS (ZnO-hybrid LEDs). As can be seen from the figure the room temperature PL spectrum of the PFO (without ZnO NRs) reveals two violet emissions at 428 nm and 450 nm, and a weak broad blue-green emission centered at 480 nm. The room temperature PL spectra of the ZnO-hybrid LED consisted of ultraviolet (UV) emission centered at around 380 nm, PFO violet/blue emissions and two deep level emission (DLE) bands from ZnO NRs. These are the green band centered at 509 nm and the red band centered at 655 nm. The origin of the DLE has been controversial for decades. This is probably one reason for the debate, since different samples have different defect configuration due to the different growth method used for each case and also the different growth conditions. The probable radiative intrinsic deep level defects in ZnO are oxygen vacancy (V O ), zinc vacancy (V Zn ), oxygen interstitial (O i ), zinc interstitial (Zn i ), oxygen anti-site (O Zn )), and zinc anti-site (Zn O ) [2]. the samples grown at 75 o C has a FWHM of 12.8 nm for the UV emission and possesses a UV emission with higher intensity compared to the previous sample. Finally the ZnO NRs grown at 50 o C possesses a UV peak with a FWHM of only 8.8 nm and the intensity of the UV emission is even higher than the other two ZnO NR samples. The trend of the FWHM and intensity of the UV emission as the growth temperature is decreased is in fact consistent with the trend of the intensity of the DLE band. The DLE band intensity decreases as the growth temperature is increased. The ratio of the NBE/DLE is taken as a measure of the optical quality, with a large value indicating an improved crystalline quality [2]. From Figure 4 it is clear that this ratio is decreasing as the growth temperature is increased. In fact the intensity of the NBE for the ZnO NRs grown at 50 o C is enhanced by three times as compared to the sample grown at 95 o C. This implies that the optical quality of the grown ZnO NRs is improved as the growth temperature is decreased. Some of the samples grown here were further processed to fabricate LEDs. The details of the fabrication process can be found in [10]. In the present paper we show results from ZnO NRs/p-polymer and ZnO NRs/p-GaN. For the ZnO NRs-p-polymer hybrid junctions we used different combination regarding the polymer side of the hybrid junction. Since the polymer emission is important as it can influence the emitted wavelength, it will be important to fabricate LEDs using ZnO NRs combined with different polymer. We have fabricated three different LEDs on these structures and these three LEDs where fabricated using PFO (blue emitting polymer), MEH PPV (red emitting polymer) and their blend. Figure 5 show the EL spectra of the three LEDs under forward bias. As can be seen the three different LEDs yielded a broad spectrum but there is a large shift in the center of the emitted spectrum. LED A (ZnO NRs/PFO) has an emission peaks at 455 nm, 470 nm and 510 nm. The first two blue peaks corresponds to the PFO and they are due to the excitonic emission and its vibronic progression from non-interacting single chains [11], and the third peak which extended from 510 nm to 610 nm has been attributed to various defects sources that is either point defects such as oxygen vacancies V O , zinc vacancies V Zn , or due to recombination of electrons with surface states in ZnO [12]. LED B (MEH PPV/ZnO) has an extended emission from 575 nm to 750 nm with a peak centered at 605 nm which corresponds to the MEH PPV due to the PPV backbone that arises from the relaxation of excited -electrons to the ground state [13]. At the MEH PPV/ZnO interface the potential barrier for hole injection is 2.5 eV, so few holes can be injected from the MEH PPV side into the ZnO NRs. Hence holes are confined to the MEH PPV layer. The potential barrier for electrons is 1.2 eV and the mobility of electrons in the ZnO is higher than that of holes in the MEH PPV so the recombination primarily occurs in the MEH PPV side. This is the reason why a sharp peak appeared in the EL spectrum and it is originating from the MEH PPV. For LED C (formed by the blended PFO and MEH PPV) the EL emission emerged at ~450 nm and extended to 750 nm i.e. covering the entire visible emission. The PFO emission peak is clearly visible at ~455 nm, while the green emission from the ZnO DBE along with the orange-red emission from the MEH PPV are completely intermixed yielding a broad visible emission. In the blended polymer layer the smaller bandgap MEH PPV lies inside the PFO which has a larger band gap, in such situation, the excitons, electrons and holes will all move preferentially onto the lower band gap material. This can be exploited by blending small amount of MEH PPV with the larger bandgap polymer, i.e. the PFO in this case. It is seen that even at very low concentration of the MEH PPV in the blend, the energy transfer is predominant and the device gives intense emission from the MEH PPV.

The PFO emission is relatively low but is still high enough to influence the colour quality of the device. We found that a blend ratio of 9:1 regarding the PFO: MEH PPV yields an intrinsic white light with good colour quality.

In order to distinguish the white-light EL spectra components and the contribution of the PFO emission we used a Gaussian function to simulate the experimental data as shown in Figure 6 (a). The simulated EL spectrum shows six different emissions at 449 nm, 471 nm, 501 nm, 531 nm, 583 and 634 nm which are associated with violet, blue, green, yellow and red, respectively. The emissions at 531 nm (green), 583 nm (yellow) and 634 nm (red) are associated with intrinsic deep level defects in ZnO. The green band is the most investigated and most debated band in ZnO. The green emission has been explained to be originating from more than one deep level defect; V O and V Zn with different optical characteristics were found to contribute to the broad green luminescence band [14]. The yellow emission was proposed due to the transition from conduction band to O i and red emission due to transition from Zn i to O i [14]. However no consensus has been reached regarding the origin of the different observed colors, partly due to the different defect configuration in different samples [15]. We have many states in the ZnO band gap due to native defects in ZnO. The energy states of various native defects had been calculated or experimentally measured in the band gap of ZnO. The emissions at 449 nm 471 nm and 501 nm are due to exciton emission of PFO. The minute contribution in green emission from PFO is due to the formation of excimer [16]. These PFO related violet and blue emissions combined with ZnO defect related green and red emissions results in the white-light emission. The Figure 6 (b) reveals the room temperature EL spectra with all emission covering the whole visible spectrum.

To investigate the emitted color quality of the presented hybrid inorganic-organic LEDs color chromaticity coordinates were plotted. Figure 7 displays the chromaticity coordinates of the three different hybrid LEDs. For the first LED, i.e. LED A fabricated using ZnO NRs/PFO, a bluish white impression is achieved with correlated color temperature of 11000 o C and having color coordinates of (x=0.2115, y=0.3724). In the case of LED B the coordinates were x=0.5461 and y=0.3724. Finally for LED C fabricated by using the blended polymer (PFO and MEH PVV), the color coordinates were 0.3619 and 0.47964, i.e. a white impression is achieved in this case.

At present we are designing different ZnO nanorods based LEDs for efficiency measurement. Although no output power data are yet available for ZnO nanorods based LEDs, the published internal quantum efficiency (IQE) results obtained from ZnO nanorods are promising [17,18]. ZnO nanorods grown by a similar low temperature process showed IQE of 28% when measured by a He-Cd laser under excitation power density of 22 W/cm 2 at a wavelength of 325 nm [17]. While a higher value of 52% for the IQE of the near band edge emission was report for ZnO nanorods grown by the metal organic chemical vapour deposition (MOCVD) [18]. While performing our measurements in dark, a clear white impression was clearly visible with the naked eye from the presented LEDs.

Conclusion

In this paper we have shown that lowering the growth temperature when using the aqueous chemical growth aqueous can lead to better crystalline quality contrary to what was believed before. Three times improvement of the intensity of the UV emission is observed when comparing ZnO NRs grown at 50 o C to those grown at the commonly used growth temperature of 95 o C. Moreover, lowering the growth temperature will also be accompanied by morphological changes on the ZnO NRs. These changes are due to the fact that the higher the growth temperature the higher the nucleation radius of the seed particle(s) and hence this morphological changes are manifested in longer and thinner ZnO NRs as the temperature is reduced further. Moreover, this low temperature growth (at 50 o C) is demonstrated on crystalline as well as amorphous substrates (flexible plastic) and it turns out that the crystalline quality is not affected by the substrate. Light emitting diodes fabricated from ZnO NRs on both categories of substrates are demonstrated. On the amorphous flexible plastic substrates, hybrid devices using p-type organic semiconducting polymer emitting in the visible region are fabricated and characterized. The effect of the polymer emission on the LEDs performance was investigated for blue emitting polymer, red emitting polymer or a blend of both hybrids junctions. The results indicate that the polymer emission can shift the center of the broad band effectively. The best white impression was obtained when using blended polymers with a low concentration of the lower bandgap polymer. The presented results of the hybrid organic-inorganic structure on flexible substrates (plastic) provides a potential for the development of intrinsic white light sources compatible with existing armature. EL Intensity (a.u.)

Figure captions

Wavelength (nm)

Figure 1a -

 1a Figure 1a-b show a typical SEM of ZnO NRs grown by the above described aqueous chemical growth at 50 o C on flexible plastic coated by a bi-layer of top Ag thin film of a thickness of about 125 nm, and a bottom layer of PEDOT:PSS polymer layer of about 50 nm thickness. As can be seen the grown ZnO NRs were grown with good

Figure 4

 4 Figure 4 shows the room temperature PL spectra of different ZnO NRs grown at 95 o C, 75 o C and at 50 o C. In general the main feature of all three PL spectra is the

Figure 1 :

 1 Figure 1: SEM micrographs of ZnO nanorods grown at 50 o C following ACG procedures on (a) Ag thin film coated flexible plastic and (b) on PEDOT:PSS coated flexible plastic foil.

Figure 2 :

 2 Figure 2: XRD of the ZnO nanorods grown at 50 0 C on Ag coated flexible plastic substrate.

Figure 3 :

 3 Figure 3: Room temperature photoluminescence (PL) spectrum of the ZnO NRs/PFO hybrid structure and the PL spectrum of the PFO on PEDOT:PSS plastic.

Figure 4 :

 4 Figure 4: Room temperature photoluminescence (PL) spectra of ZnO nanorods grown at 50 0 C, 75 0 C and 95 0 C on Ag coated plastic.

Figure 5 :

 5 Figure 5: Electroluminescence (EL) spectra of the three different LEDs fabricated using ZnO NRs/p-polymer hybrid junctions, the spectrum labeled by A is for the case of using the PFO, while B indicates the case of using the blend of PFO and MEH PPV, and finally C indicates the case of using the MEH PPV polymer.

Figure 6 :

 6 Figure 6: (a) Room temperature EL spectrum with Gaussian fitting (b) The EL spectrum showing the whole visible range as distinguished by the Gaussian fitting.

Figure 7 :

 7 Figure 7: Chromaticity diagram (CIE coordinates) of the three hybrid LEDs.

Figure 1 :

 1 Figure 1:

Figure 2

 2 Figure 2:

Acknowledgement:

Partial financial support from Functional Materials project at Linköping University is highly appreciated.