Selection Tips for Choosing the Right LB System for You

Design Concepts, Features and Performance

Contents

1. Abstract.
2. Design Concept and Features.
3. Design of the different modules of the system:

• Trough and barrier.
• Temperature control.
• Surface pressure meter.
• Mechanical modules.

    4. Conclusion.

1. Abstract

An analysis of the main design concepts used in the manufacture of today Langmuir-Blodgett (LB) film deposition systems is presented . A critical comparison is made between the 4^th generation of LB systems (500 series) designed by Advanced Technologies Ltd. (AT) and the last modifications from its major competitors: Nima Technologies Ltd. (U.K.), KSV Instruments Ltd. (Finland), and Nippon Laser and Electronics (NLE) Lab. (Japan). Analysed are all major components: trough and barrier, temperature control, surface pressure meter, barrier moving mechanism, film deposition system. Our aim is to help our customers with competent advice on choosing the correct instrument for their needs. We wish to draw attention on specific equipment problems and trade-offs, which are not usually discussed in equipment leaflets, and which contribute to the precision of operation and ease of work.

500 series from ATL is the best specified LB film deposition system currently on the market. It is designed as a top performing equipment for advanced research needs. Several new trends are incorporated in the design: modularity Ц one system can work with different troughs for the different experiments, or can be upgraded; open and compact design for easy integration with other research equipment like microscopes, spectrophotometers, etc.; built-in features for complete control of experimental conditions and work with different type of compounds in different experiments. This is the only commercially available system with built-in temperature control (option). With lower cost and improved precision compared to external thermostat with cooling features, the built-in temperature control allows complex experiments like isobars and isochores or thermal cycling at given position of the phase diagram for film quality improvement. The temperature range in the 500 series is considerably improved, compared to 400 series and now it is from 5 to 40 ^OC, software limited; the accuracy of control is ▒ 0.05 ^OC within 1 hour from experiment start; the desired temperature is usually reached within 10 minutes. Most importantly this is the true temperature in the water subphase measured in the dipping well and not the temperature anywhere else where we have shown that the errors can be as high as 10 ^OC.

We were able to produce these highest quality very reliable systems and at the same time maintain competitive prices on the market. How did we achieve this? For two reasons. First we have started the design of the 500 series in 1997 while the models of our competitors are from late 80s or early 90s. So we were able to use new components and feel the new trends. Second, because of the specific situation in this country (Bulgaria was called the Silicon Valley of the late communist countries, but due to current transitions many top trained engineers were left without properly paid and challenging work) we were able to hire quality engineers none of our competitors can afford to hire. For example the chief mechanical designer of the 500 series has several instruments still flying on the space station Mir.

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The main design concepts in the 500 series are:

• Top specified extremely versatile system for advanced research investigations;
• Compact and open design for easy system integration with other research equipment;

• Product superiority as regard precision, built-in features, ease of operation;
• Modular design: different troughs can be used for the different experiments and can be changed within seconds by a patented method; completely modular electronic block; easy upgrade from Langmuir to LB system;

• Extremely high reliability through the use of last generation electronic components, special protective components, highest grade motors and other mechanical components;
• Ease of serviceability for low downtimes in the unlikely case of malfunctioning.

Features as regard precision:

• Precise surface pressure meter with long-term drift and noise of ▒ 0.03 mN/m (with additional filtering ▒ 0.01 mN/m). This is the only surface pressure meter implemented in an LB system which can measure with a true accuracy of 0.01 mN/m. This is in part due to the unique 22 bit resolution (for 100 mN/m range the internal resolution is 0.00005 mN/m) Analogue-to-Digital Converter with 1000 measurements per second;
• Precision manually driven vertical positioning of the surface pressure head for easy calibration against water or other compound with known surface tension;
• Constant temperature control to within ▒ 0.05 ^OC after one hour of stabilisation with the optional built-in temperature controller with no commercial analogue;
• Constant pressure maintained to within ▒ 0.02 mN/m through a special algorithm;
• Maximum 10 readings per second of the parameters (surface pressure internally is read 20 times per second) for high time resolution measurements. Averaging filter with variable length can be applied to data;
• A choice of 8200 velocities for the barrier and substrate lift in the range 0.004 Ц 8 (options possible) mm/sec precisely maintained without generating vibrations by a patented solution;
• Three compression methods: with constant velocity; equilibrium compression with pressure steps and waiting for pressure to stabilise with time; constant strain compression with linearly decreasing velocity;
• Position of the barrier and substrate lift measured with absolute accuracy of 0.005 mm which is equal to 0.002 % of trough length;
• Temperature measured with a resolution of 0.01 ^OC and absolute accuracy of 0.5^OC;
• Built-in temperature compensation of surface pressure due to the change of water surface tension with temperature.

Features as regard integration with other equipment:

• Even for a system with built-in temperature control the height from system bottom to trough top between the barriers is 32 mm (for troughs with no borders). This is less than the distance between the microscope objective and the microscope table so the system can be placed directly on the microscope table. This is the only full size commercially available system that can do that.
• Surface pressure module and LB film deposition module are only 35 mm in thickness and smallest side dimension. Thus is minimised the disturbance of laminar flown air;
• The system has the smallest footprint on the market with only 298 cm² surrounding the trough (without the LB film lift). The standard trough itself is with dimensions 30.4 x 20.4 cm and effective trough area of 600 cm²;
• The 500 series has a unique hand-held panel (option) with LC display on which all important parameters during the experiment are displayed, large LED display for over the room view of the most important parameter (can be changed by a press of a button) and specially designed sealed keyboard for manual control of the system. The standard cable is 2 m and the panel can be placed on any convenient place. A longer cable can be ordered for work from another room;
• For the different experimental setups different troughs can be used. The exchange is within seconds without loosing the accuracy of measurements;
• We constantly manufacture equipment according to customer specifications or needs. Sometimes this is free of charge.

Features as regard functionality and ease of work:

• Possibility to measure isobars and isochores with the optional built-in temperature controller. No need to handle another instrument as the external water bath controller. Multiple cycles of increase and decrease of temperature at a given pressure or mean molecular area;
• 90 degree tiltable film lift (option) with readout for various modes of deposition. Possibility to measure static or dynamic contact angles by the method of Adams;

• Windows type GUI (graphics user interface) menu driven software. Easy to learn and use but powerful;

• Online and context sensitive help;
• For ease of work the setup parameters are divided in 3: the parameters which are specific for the experiment (typically only 6 or 8) are separated; setup almost constant parameters (15) do not have to be filled every time; parameters which are typical for the hardware or which optimise regulators (22) are protected for accidental change by password. Default values with which any experiment can be started are provided. Check for parameters to fall within sensible limits is performed and if not warning appears. All setup parameters are stored with the file of experimental data for subsequent view. On opening a data file the experiment specific parameters are automatically loaded so an experiment at same conditions can be started by only changing the file name. Two lines for comments are provided;
• Three parameters for wait times before the experiment proceeds: for the evaporation of solvent; for temperature stabilisation; for constant pressure or area film stabilisation;
• Individual film lift velocities for the first layer and for movement up or down;
• Real time full screen view of experiment progress. The screen view follows the operation in progress, e.g. when LB film deposition is performed first p - A isotherm is displayed and when the lift starts to move Transfer ratio vs. lift coordinate is displayed. Autozoom of surface pressure axis for accurate view. Experiment can be: a) halted for change of a few parameters like compression velocity or targeted constant pressure and then the experiment continues; b) stopped which stops all motions but data recording continues; c) quitted which saves the data and exits the experiment.
• Powerful off line data analysis. Full screen graphics for maximum clarity. White or black background for decrease of eye fatigue. Up to four Y-axes can be selected as a function of the X-axis. Up to 4 files can be displayed simultaneously in same coordinates. All possible combination of coordinates, including the 3 additional inputs, is possible. Calculation of compressibility modulus as a function of mean molecular area. Multiple zooms. Precise readout of the cursor pointer for identification of points on the graphics. Selection between 0 to 128 subsequent point averaging.
• The analysed data can be inserted directly into your document through the Windows Clipboard and printed. Alternatively the binary files can be converted to ASCII format with headers for point by point observation (included are all experimental parameters) or without headers in columns for direct input in graphical or analysis programs like Excel, Grapher, Mathlab, etc.
• Full range of lower cost (compared to the competitors) accessories so that the system can be precisely tuned to the customer requirements.
• 3 free analogue inputs for high accuracy measurement of signals. Optional one additional serial input.
• Possibility to emulate experiment with a pre-recorded data files for educational or advertising needs.

Features as regard reliability and serviceability:

• The system is developed around latest generation highly integrated electronic components. The decreased number of components leads to increase in reliability;
• Electronic hardware is developed around standard size modules which makes easy to detect and repair the potential problem or upgrade the system;
• Highest quality DC servomotors from Maxon (Switzerland) are used and further steps for increasing their life are taken;
• Highest quality stainless steel linear slide guides from Nippon Bearing (Japan) are used;
• All other components used are from world leading companies like Bourns (USA), Gates (England), etc.
• The estimated mean time between failures (MTBF) is 30000 hours. This means that if the system is used 10 hours a day 300 days per year the first problem may be expected after 10 years. We have heavily used equipment for 5 years with no problems so far.
• The instrument has minimum electromagnetic emission. So no disturbance of other sensitive laboratory equipment can occur. Highest quality linear power supplies are used and linear motor and temperature control circuitry. So there are no powerful sources of noise. Even the power transformer in the temperature control unit is torroidal type for further decrease of emission.
  
  

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The wide variety of LB film applications requires flexibility in the geometrical design of the trough. Sometimes special shapes and sizes are needed. The use of original rectangular trough with moving barrier sliding over the edge of the trough offers greatest flexibility in geometrical design of the troughs. This is the design used by ATL, KSV, NIMA, and in some models of NLE. In band and moving wall types of system, the band defines the trough shape while acting as a barrier. In such a system the trough geometry is fixed and cannot be changed to fit special applications. Further on the band and pulley mechanisms are much more difficult to clean. The band is pushed outwards by film pressure. Film collapses near corner post with condensed film. The mechanics is complex and causing vibrations. In moving wall compression (used by NLE) usable film area is determined by width of the substrate, moving wall scratches sides of the substrate and there is no possibility for dipping of circular substrates. In circular trough (earlier version and still offered by NIMA) the pressure measurement is unstable due to uneven pressure distribution caused by the difference of film speed at inner and outer trough perimeter.

In ATL we use almost square geometry (275 x 218 mm for the standard trough, 600 cm²) to minimise film flow and the friction with the side walls. NIMA has an aspect ratio of 3:2 and 600 cm². KSV has narrower design (510 x 150) which increases compression ratio. High compression ratio, defined as the maximum to minimum usable area is important when working with films in which surface pressures starts to rise at high mean area per molecule. The ATL embeds the dipping well in the back wall (in some models) to increase the compression ratio. In ATL the compression ratio is between 7 and 10 depending on the method of compression and model and is limited by the space necessary for the Wilhelmy plate. To illustrate the importance of compression ratio lets have the example of arachidic acid. With minimum mean molecular area (mma) of 19 A ² in a trough with ratio of 10 the compression can start at 190 A ² mma. High compression ratio is important in surface potential measurement because changes in the potential are observed at large areas earlier than changes in pressure.

ATL and NIMA systems have a uniform water depth of nominally 4mm over the whole trough area (the minimum stable depth for a water layer on a hydrophobic surface). These are ideal for biological investigations of the binding of proteins to model membranes, because the minimal subphase volume economises on protein which is often precious. Smaller subphase volume also improves the temperature control giving faster times for reaching the desired temperature and more homogeneous temperature distribution. KSV have a trough depth of 10 mm, NLE Ц 15 mm. Troughs for film deposition have a larger volume of subphase, associated with a region in which the water is much deeper. This dipping well allows deposition of the monolayer onto solid substrates by the vertical LB dipping technique. In ATL we offer a shallow well of 14 mm depth (35 x 35) which in most cases is sufficient for todayТs experimental techniques (all kinds of microscopy and spectroscopy, X-ray diffraction, elipsometry, etc.). The larger dipping well is 42 mm in diameter and 45 mm in depth and is ideal for deposition on microscope slides (26 x 76 mm). Larger wells can be manufactured on request but with the trade off of temperature accuracy and larger subphase volume. Competitors usually offer larger dipping wells because they have never fought the problem of even temperature control. It takes 1 hour for the water to stabilise in the 42 x 45 mm well and it is exactly where the temperature accuracy is important. Because on top of the well the deposition takes place and nearby is measured the surface pressure. The longer the time for experiment the more contamination falls from air. The subphase volume in ATL even for the system with large dipping well is only 0.4 l. This is among the smallest on the market.

Trough and barrier material

PTFE (polytetrafluoroethylene), known with its commercial name Teflon, is generally accepted as the best material for LB systems because it is inert, highly hydrophobic, and easy to clean with organic solvents, acids and bases. However there are differences on how it is used. KSV uses form-sintered PTFE with integrated dipping well. Thus no glue is used for the sealing of the dipping and trough and there is no risk of contamination. But glue has to be used when optical window is inserted in trough bottom. Further on we believe that the small companies that perform this form-sintering do not have highest level of technology, necessary for obtaining pure and porous free PTFE. That is why we at ATL prefer the use of highest quality commercial PTFE solid blocks from Norton Performance Plastics (USA) which is among the largest manufacturers of PTFE products. Careful selection of glue ensures no contamination and high quality seals. NIMA also uses this approach. NLE and other Japanese manufacturers use PTFE coating on Aluminium. This is less expensive and ensures lower thermal losses during thermal control. But it is known (also from LB literature) that there is porosity and impurities caused by the spraying process, and contamination with metal ions are observed. They cannot be repaired in case of scratches.

Barriers are usually also manufactured from PTFE. This is the case of ATL and NIMA. PTFE has the advantage that can be cleaned with everything, including strong oxidising agents for removing contamination. Although hydrophobic PTFE barrier also drags the water upwards though to a lesser degree than the alternative hydrophilic barrier. However the leaks are associated with three contact point of barrier, trough edge, and air-water interface. KSV offer barriers from the hydrophilic material Delrin (Acetal Homopolymer). It decomposes in short time by strong acids and alkalis but can withstand chlorinated hydrocarbons for cleaning. ATL offers this as an option for its barriers. The use PTFE coated belts by NLE is subject to problems associated with belts mentioned above.

3.2. Temperature control.

This is the only commercially available system with built-in temperature control (option). With lower cost compared to an external thermostat with cooling features the built-in temperature control allows complex experiments like isobars and isochores or thermal cycling at given position of the phase diagram for film quality improvement. The temperature range in the 500 series is considerably improved, compared to 400 series and now it is from 5 to 40 ^OC, software limited; the accuracy of control is ▒ 0.05 ^OC within 1 hour from experiment start; the desired temperature is usually reached within 10 minutes. Most importantly this is the true temperature in the water subphase measured in the dipping well and not the temperature on the metal plate below the Teflon trough or in the remote thermostat. We have shown that in order to maintain the temperature at 5 or 40 ^OC the metal plate below the trough should be kept at about 8 ^OC lower or higher temperature. So all published isotherms at extreme temperatures measured so far without in-trough thermometer should be regarded with great care.

Constant temperature maintained at 38 ^OC.

Constant temperature maintained at 7 ^OC. After one hour from reaching the desired temperature the accuracy is ▒ 0.1^OC improving with time.

3.3. Surface pressure meter

Two basic designs exist of the Wilhelmy type surface pressure meters. One of them uses linear position sensor (usually linear variable differential transformer (LVDT)) for measuring the displacement of the Wilhelmy plate. The measuring element (the anchor in the case of LVDT) is attached to a spring so this becomes a force measuring unit. The advantages of this design are that it is not breakable, linearity is higher, the noise is lower, and the unit is more reliable. The drawbacks are the larger thermal drift if the environment temperature changes, fatigue in the spring which may require recalibration, and the change of the Wilhelmy plate position. The last drawback is solved with today's high sensitivity designs so that total displacement at full range can be made lower than 0.1 mm. The results presented below are with this type of design.

The other design utilises an analogue microamperemeter on the indicator arm of which is attached the Wilhelmy plate. The arm is illuminated by a LED and throws shadow on a couple of photodiodes. The signal from the photodiodes is used to maintain the arm in constant position and the surface pressure is proportional to the current which flows in order to maintain that position. The advantages and disadvantages are discussed above. The major disadvantage is the fragility of the system. In fact the more accurate is the system the more fragile it is. Some of our customers had problems with this. All our competitors use this design. We also manufacture it and use it in our tensiometers.

The performance of our system is shown in the specifications and is demonstrated below.

Noise in water ▒ 0.005 mN/m. 10 meas/sec. 2 minutes.

Averaging with K = 7.

Long term drift. 4 hour experiment. One measurement every 5 seconds. No additional filtering. Drift is ▒ 0.04 mN/m. With additional averaging the drift is ▒ 0.02 mN/m.

Drift in water for 6 hours. ▒ 0.05 mN/m. No filtering.

Reproducibility of surface pressure. 6 cycles of compression and decompression. From upper curve to lower curve: 1^st compression, 2^nd compression, 3^rd compression, 1^st decompression, 2^nd decompression, 3^rd decompression.

Reproducibility.

A difference of 0.125 mN/m of the inflex point of Arachidic Acid

Reproducibility.

0.05 mN/m difference at low pressures. K = 5.

Reproducibility and noise in a real experiment.

Additional averaging on data points was applied K = 7.

Liquid-gas coexistence region is clearly visible. It is seen also that compression velocity was high (observed also at liquid Ц solid coexistence).

Constant surface pressure control at 5 mN/m. On the inset is zoomed surface pressure. Accuracy is better than ▒ 0.03 mN/m.

Detailed comparison with other companies is not possible because no one from our competitors gives detailed information on the performance of their surface pressure meters. The parameters shown are the range Ц typically 0 Ц 100 mN/m, and a parameter, which they call accuracy Ц typically 0.004 mN/m. But this is not accuracy. This is resolution and it is obtained by dividing the range to the Analogue-to-Digital Converter resolution. NIMA and KSV use 16 bit resolution ADC. Compare this to ours 22 bit resolution ADC. Of this 16 bits, one is used for the sign, and remaining 15 bits give a resolution of approximately 1:32000. If you divide 100 / 32000 = 0.0031 you obtain the specified by them accuracy. But this is far from being true. First from theory of measurement it is known that you should have at least 4 times higher resolution to avoid discretization noise. ThatТs why in all our internal calculations we use 18 bit resolution. More importantly, the actual accuracy is limited by noise, long term drifts, repeatability, and nonlinearity. These parameters are specified for every analytical balance and the surface pressure meter is largely an analytical balance. But they are not specified by our competitors. It is true that in Langmuir film experiments absolute accuracy is limited by external factors like wetting angle, calibration accuracy, etc., to about 0.1 mN/m. But relative accuracy, i.e. a step or transition in an isotherm, is much higher. With our system we demonstrate measurement of liquid-gas coexistence region which is in our case (NBD Ц DPPE molecule) only 0.1 mN/m above the gas phase. From what we know about our competitors no one can measure this.

Demonstration of Isobars experiments.

7 temperature cycles between 6.5 and 35.5 ^OC. The pressure was kept constant at 30 mN/m. On the lower curve is shown the change of area with temperature. This experiment would not be possible without the built-in compensation of water surface tension change with temperature.

Change of Mean Area per Molecule with temperature. Relaxation of monolayer with temperature cycling can be observed. Its thermal expansion coefficient can be calculated. Excellent reproducibility can be noticed.

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We use the best possible design for both the barrier module and the lift module. This is the use of DC servomotors with analogue tachometer feed back loop for maintaining constant velocities. NIMA uses DC motors but the tachometer feed back is only an option, that is why their range is limited to 1:200. KSV uses DC motor only in their lift but with digital feedback Ц hence the dynamic range is limited to 1:100. For their barrier mechanism they use stepper motor in microstep operation. But when you specify a range of 1:40000 this means that the motor makes a step and then waits for several seconds. This inevitably generates surface waves and vibrations. NIMA and KSV use 12 bit Digital-to-Analogue Converters for motor control. Compare this to ours 14 bit DACs. We have 4 times better resolution which we can utilise by our mechanical design.

In our case we use linear tachometer feed back as opposed to pulse-width modulation control. For small motors this is a far superior solution which increases motor life and decreases unevenness of motion. This in combination with the most precision slide rails and precision position sensors, attached to the end stage for true position readout, produces the best mechanical design. This is important not only for smooth, even, vibration free motion with large velocity range. This is important also in the surface pressure control in which the barrier acts as a feed back.

Accuracy of barrier motion. The area decrease versus time is an absolute straight line with no deviations, indicating unevenness or vibrations. The best way to test mechanics is to put your finger on a quickly moving barrier or lift. You can feel the smooth motion and you can see the difference with competitors.

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4. Conclusion

Here we have presented proof that the 500 series of Langmuir-Blodgett film deposition equipment from Advanced Technologies Ltd. is the most accurate and versatile system on the market. What is more, our system is at least 10 times more precise in all important parameters than the comparable system of our major competitor Ц NIMA Technology (U.K.). Compare:

• the surface pressure sensor nonlinearly: 1 % NIMA Ц 0.1 % ATL; look at our noise of only 0.005 mN/m. The real values of our competitors is not known.
• accuracy of barrier position measurement: 1 % NIMA (multiturn potentiometer) Ц 0,002 % ATL (encoder);
• accuracy of temperature control: 0.05 deg. C ATL Ц 7 deg errors at 5 or 40 deg C if no feedback thermometer is provided from the trough dipping well to the thermostat, which is not usually done.
• velocity dynamic range: 1:200 NIMA Ц 1:10 000 ATL
• resolution of the used electronics: for the ADC: 16 bit NIMA Ц 18 bit ATL out of 22 bit ADC; for the DACs: 12 bit NIMA Ц 14 bit ATL.

KSV (Finland) system is better compared to NIMA. Very little official information is provided so what can be said at this moment is that our system is at least as precise as KSV in points 1 and 2 above. The rest 3 points are absolutely true again. And even the low cost models of KSV are approximately 2 times more expensive than a 500 system.

Our software is more powerful and with more features. All possible data analysis functions are built-in. Export of the data file and setup parameters in ASCII format is available for data import in other software. The software has self-test and self-calibration features to make the system easy for work. Software protection of the system is combined with hardware protection in all modules. This adds to system reliability and to fault tolerance.

We are currently establishing contacts with leading companies to distribute our products in the corresponding countries. This will give our products unparalleled support and our customers - piece of mind.

Last but not least, our prices are at least 10 % lower than the lowest prices on the market Ц the NIMA prices. And NIMA in most countries operate without local distributor so it can not provide the customersТ support that our distributors can.

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